Basically, they all rock. And here's why.

First and most important, they provide one of only a handful of FPS experiences I've ever played where the compulsion to unearth more of the storyline is far stronger than the simple compulsion to kill more stuff.

Sure, the basic tale is classic sci-fi nonsense, but it's told with such panache and, more importantly, character that you actually find yourself caring about it. Not in a 'I want to have your babies' kind of way (though actually the lead female character, Alyx, can have my babies any time she likes), but certainly to the extent that it makes the game's world feel much more tangible, with the result that you feel much more a part of it.

Three elements make the storyline work so well. First, there's the basic script, which shows constant imagination and wit.

Then there's the voice acting, which is uniformly excellent and really helps build the game's characters.

And finally there's the brilliant way that practically the entire story is told without recourse to cut scenes, meaning you're never lifted out of the thick of the action.

Of course, an FPS can't get by on story alone. Its shooty bits also have to be deeply satisfying. And that's definitely the case with the Half-Life games.

This is partly due to some pretty solid enemy AI, whereby they're never unfairly good or stupidly bad. But it's also partly due to the satisfyingly diverse arsenal at your disposal and the sheer variety of the combat scenarios on offer.

For instance, one minute you'll be sneaking around cramped corridors, shotgun at the ready for some close and personal encounters with Combine/Human zombies, and the next you'll be zipping about on a boat with a mounted gun picking off helicopters, gas-masked soldiers and hostile army vehicles from a distance without even taking your foot off the gas.

Or maybe you'll be using a crane to take enemies out. Or running over things in a car. Or shooting down a helicopter with a rocket launcher. Or cutting mutants in half with a gravity-propelled saw blade...

This latter hilarious murderous activity introduces us to another thing that makes the Half-Life games so great: the gravity gun. This fab device can pick up and hold pretty much anything, and can then propel whatever's being held forward with enough force to take out anything in its path.

This really does make for a wonderfully novel way of 'taking out the trash' compared with just pumping enemies full of lead all the time. If you're really quick you can even use the Gravity Gun to catch some of the nasty stuff the enemy fires at you, such as grenades, and send it pinging back at them. Mwuhaha!

Just occasionally, at least during Half-Life 2 and Half-Life: Episode One, the odd level - such as the Root Kanal/Water Hazard bits in the main game - drags on a bit too long. But for the most part Valve's sense of timing and drama is impeccable throughout all three games, leaving you knowing that something new and great is nearly always just round the corner.

This being the Xbox 360, there are also Achievement points to consider. And I'm happy to say that while some of the 99 achievements on offer are diabolically hard to get, they nearly all combine a mischievous sense of humour with a really canny understanding of what makes Achievement obsessives tick.

For instance, one achievement is rewarded for managing to cross an enemy-infested beach without once touching the sand (by carefully laying side by side strips of corrugated iron and boxes). Or there's one for pinning a soldier to a billboard using a crossbow. Then there's the one called 'flushed' for killing an enemy with a toilet. And one for using cars to crush 15 Antlions. And the Pinata Party one for finding all the secret 'web caches' in Episode Two. And the Zombie Chopper one for completing the tough Ravenholm level using only the Gravity Gun and the available supply of saw blades.

One more achievement I just have to mention is my personal favourite: Little Rocket Man, awarded for going through the genuinely painstaking agony of carrying around a garden gnome in your gravity gun for pretty much the entire duration of Episode Two, so that you can pop him into a rocket at the game's end!

As you can see, some of the achievements on offer are there to tempt you to find more innovative ways of dispatching bad guys than just pointing your biggest gun at them, while others are designed to reward deeper exploration of the games' environment than you would probably otherwise bother with. But just about all of them ultimately ensure that you get far more out of the games and play with them for far longer than would be the case without them.

It's worth finishing the discussion of the Half-Life games by saying that added together, the three of them should deliver what I'd estimate to be around 30-35 hours of gaming pleasure - even without going all-out for the various achievements on offer.

That makes for a startling amount of hugely satisfying first person shooting in one affordable package. And we still haven't even touched on Portal and Team Fortress 2 yet. Portal is, in some ways, the most lightweight part of the Orange Box proposition, in that once you get the hang of it you can probably get most of it done in three or four hours. But it's also stunningly original, fiendishly addictive while it lasts, and the proud owner of one of the most unforgettable computer game characters ever, the robotically warped Glados.

Glados is the computer guide who 'helps' you through your time solving puzzles in the Portal training facility, combining the odd bit of genuinely helpful advice with increasingly insane banter that makes 2001: A Space Odyssey's HAL sound positively normal by comparison. And she helps create what feels like the perfect half-comic, half-sinister backdrop for the main Portal gameplay to appear against.

To succeed in cracking Portal's increasingly devious puzzle chambers by manouvering yourself to their exits, you'll really have to think in a 3D, spatial relations way no other game has ever required before. While also sparing a thought for such heavy-sounding stuff as gravity, momentum and geometry.

If this all sounds frighteningly nerdy and beyond your brain power, trust me when I say that while it may seem bewildering for the first half hour or so, you suddenly get the hang of it. And once you do you just can't wait to experiment more with what your trusty portals - not to mention your delightful 'Companion Cubes'... - can do for you.

It's worth adding, too, that while most gamers should be able to finish the main Portal game, it does also boast some Advanced levels that open up after the main game is complete, and some of these are so brain-bending we reckon even Carol Vorderman might be left scratching her perfectly coiffured bonce.

Valve's use of Achievements for Portal is excellent too, offering rewards for completing chambers within restrictions such as a limited number of footsteps or a limited number of portals.

Still reeling from the stunning assault on my grey matter offered by Portal, it's time to hit Xbox Live with Team Fortress 2. And before I know it, my senses are reeling again, as I'm faced with a frenetically bizarre combination of cartoonish graphics, frantic action and cool comedy.

We go into more detail on the Team Fortress 2 experience in our ‘Online elements’ section, so here we’ll restrict ourselves to saying that although the online game has elements of genius about it and has enough character to earn a cult following, it’s got too many flaws in technical and gameplay terms to compete with the online world’s leading lights. The amount of lag it suffers is particularly galling.

Still, when the worst part of a five-game package still rates as ‘decent’, it’s fair to say that The Orange Box represents possibly the best gaming value we've ever seen.

Kicking off the impressive stuff is the completely cohesive feel to all five of the games. By which I mean that every element in every game’s world looks and feels completely like it belongs there, giving each game a clear and almost uniquely consistent identity that works in tandem with the unusually rich story and characters to entice you deeper into the action more fully than almost any other games we can think of.

Contributing plenty to this sense of almost organic unity to Valve's game worlds are the lighting effects, which work subtly but completely realistically to help give objects and locations a sense that they all belong in the same three-dimensional space.

The draw distance in the three Half-Life games is also striking at times, again reinforcing the sense that you’re doing your planet-saving stuff within a full-scale world. And of course, the more you believe in a world, the greater your urge to try your damndest to save it!

The Half-Life 2 graphics are also quite sharp on the Xbox, with tight edges and crisp motion devoid of the ‘blurring’ filters that let down some FPS competitors.

Finally in the positive column, Half-Life 2 generally excels with the quality of its animation, and with the way enemies and characters ‘fit’ completely believably within their three-dimensional environment. Check out any of the scenes where you have to evade and attack ‘striders’ – especially the finale to Half-Life 2: Episode Two - to see this effect at its best.

So far, so next gen. Yet while all these strengths are enough to make The Orange Box a satisfying graphical package, it can’t be considered a great one. For instance, put any of the package's five games against something truly groundbreaking graphically like Gears of War, and it’s impossible not to be struck by how rough and ready some of the textures look. Road surfaces, for instance, look almost hand-drawn at times; vehicles tend to look flat and ‘standard def’ due to a lack of graphical HD polish; and even the games’ characters look a little ‘computerised’ versus those in something like Assassin’s Creed.

Another slight glitch finds textures taking a split second longer to materialise than the graphics’ basic outlines, while colour tones are sometimes painted using rather broad brushtrokes and a slightly limited palette, detracting further from the air of realism that the graphics (for everything bar Team Fortress 2) otherwise try to create.

All in all, the slight lack of fine texturing means that Valve’s graphical engine is arguably at its most effective with Team Fortress 2, since this game’s cartoony style doesn’t actually need any great sense of texturing to look great.

There’s also plenty of bass on hand to add presence to explosions, and plenty of suitably staccato aggression in the presentation of gunfire.

Having said all that, there’s no doubt that there are plenty of games out there – Call of Duty 4, Gears of War to name but two – that serve up greater dynamics and even more effects and intensity.

But there are two final areas where The Orange Box’s audio really scores over many of its rivals. First, the voice acting in all the games is consistently superb, breathing real life into all of the countless characters you come across, no matter how minor. The vocals for Gladys in Portal are particularly memorable, and almost single-handedly give the game a sense of character few if any other games can match.

The other string to the Orange Box’s sonic bow is the imagination evident in some of the audio effects, especially when it comes to the often quite spine-chilling noises emitted by some of the game’s ‘baddies’.

What’s more, as its name suggests, Team Fortress 2 can only be played online in teams; there are none of your solo ‘free for all’ options here.

Four game types are available: capture the flag, Control Points, Territorial Control, and Attack & Defend, each with their own dedicated maps (of which there are a slightly meagre six in total) designed especially to suit particular gameplay types.

As well as choosing which type of game you want to play, you also need to pick at the start of an online game which character class you want to play as. Nine different ones are available: soldier, Engineer, Sniper, Scout, Demo man, Pyro, Medic, Spy, and Heavy Gunner. Each of these characters has widely different abilities, and it’s in these differences that most of Team Fortress 2’s potential fun lies, as the tactical options available at any time are almost limitless.

The game is also presented with some superb humour, especially in the taunting system available to each character if you want to rub in your latest kill, or just wind the opposition up.

All of the maps are well designed to encourage balanced, long games, and the weapon options seem pretty well judged for an even-handed experience, too.

However, while fun, I have to say that I found Team Fortress 2 simply too lightweight to really keep me interested in the way that Halo 3 or Call Of Duty 4’s online games keep me interested.

Far worse than this, though, is the amount of lag I experienced while playing the game online. This was unbearably bad for the first few weeks of my owning The Orange Box, but thankfully improved following the release of a patch towards the end of 2007.

However, even with the patched version of the game I’d estimate that around 75 per cent of all games I play are seriously affected by some degree of lag – an absolute nightmare when you’re talking about an experience as fast-paced as Team Fortress 2.

I’ve lost count of the amount of times I’ve been killed by someone before they’d even appeared on my screen, or the amount of times I’ve been trying to shoot at shadows as enemies in the room with me appear and disappear rather than smoothly moving around.

Trying to play Team Fortress 2 now, a few months after The Orange Box’s release, reveals that the lag problems – together, perhaps, with the paucity of maps and lack of any real incentive to keep playing - appear to have driven many Orange Box owners away from the game and into the arms of something more stable instead. The result is that it can be really quite tough to find many games to join in, especially if you want to played for Ranking points rather than pure fun. Or if you decide to set a game up yourself, you can wait an eternity for enough players to turn up. This makes the fact that you can only have one player per Xbox (there's no split-screen play) all the more disappointing.

There’s no doubt at all that there’s a really great game hiding away in Team Fortress 2. Playing via the lag-free System Link option rather than Online proves that. But there’s also no doubt that as things stand, a lack of online options, maps, statistics, leagues, graphical polish and, above all, online stability leave Team Fortress 2 feeling a bit more like a ‘trial version’ of a game rather than the finished article.