Not only the best game in the series, but one of the finest multiplayer experiences i've ever played.
The thing which stands out straight from the first level is the visual quality. Splinter Cell has never been one for ugly graphics, but Chaos Theory really does aim to impress, with some set-pieces leaving a confused feeling of "how on earth have they done that on an XBox?" Brilliantly though, the visuals aren't what make up this game; they simply accompany the excellent gameplay. Just like SC games past, you play as Sam Fisher, a government spy sent in to retrieve valuable intelligence without anybody knowing you were even there. Get killed or captured, and you dont even exist. The government has no file on you, and denys the existance of any "Sam Fisher". Tough, eh?
But you're well equipped to tackle this job, with your trusty night and thermal vision goggles, along with your all-round death machine of a rifle. Of course, the more skilled of players will attempt to get through the game as it is meant to be played, moving through missions without a single person knowing you were there. That takes some patience however, and sometimes the best strategy is to knock out a guard who wont stay out of your way, or mess with a few heads with some clever diversions. Firing your weapon gets you into a world of trouble, as guards respond to alarms, and don flak jackets and helmets when they're pretty sure there's an intruder lurking about.
The AI has improved massively over previous games. It's tough to describe the feeling of shock as a sentry tosses a flare into your genius hiding place. There are also a few more set pieces, such as the wonderful lighthouse found in the opening level, with a single guard at its top. Lots of fun to be had in situations like these.
So that's the single player down. It's splinter cell as it's always been, only now you have a trusty knife, some new gadgets, beautiful visuals and some tricky AI to get around. Great Stuff.
Amazingly though, that's not even half the reason why you should get this game. The real awesomeness comes in the multiplayer portion, which really is a godly experience to say the very least. Not only is there an entire co-op campaign specially made for two spies to move through, but we also see the return of the previous game's spies vs mercenaries adversary mode.
Simply put, this delivers the most tense, atmospheric, and damn-righ brilliant multiplayer experience around. Losing a game doesn't matter, because you know you lost because you were outsmarted, or your plan didn't go as you'd thought it would.
There are some down sides, sadly. For one, there is a noticeable down-grade in the game's visual quality as you load up the multiplayer section. Also, it's pretty obvious that the single and multiplayer portions have been developed by two entirely different studios. The menus are different in style and layout, and most irritating of all, the controls are completely different as you move from Sam Fisher to a spy in Multiplayer. It's disorientating, and generally pretty stupid when you think about how easy it could have been for one studio to simply email a set of controls to the other.
Overall however, you're looking at one hell of a game. Once you break through the single player portion, you've got a whole new co-op campaign to start on with a friend. Defeat that, and you're presented with one of the finest multiplayer experiences in existance.
Highly recommended.