Chaos Theory is an amazing technical accomplishment that surpasses its predecessors to set a new standard for it's genre
The first thing that you'll notice in Chaos Theory is that it runs off of a new graphics engine. It still retains it's stenciled look, but adds sharper, clearer textures and some incredibly fluid animations. The particles aren't the best, but other effects, like rain, look great. The environments are usually pretty dark, so you'll probably get bored of looking at night-vision by the time the game's done, but the environments look amazing none-the-less. Overall, Chaos Theory is presented well, and the level of detail is amazing considering the Xbox's hardware specs.
The audio is also great. Sound effects are forceful and convincing, and the subtle ambient noises really add depth and immersion to the experience. The voice acting is really well done, and the cool ambient sounds that let you know the enemies have been alerted to your presence work very well.
The gameplay is where the game really shines. The previous installments suffered from aggrivating trial-and-error gameplay, but Chaos Theory fixes this problem in a few ways. First, it gives you the ability to quick-save at any time, which really makes things easier. Next, you no longer fail the mission if the alarm is raised (although the AI will get a lot tougher and more alert). Finally, it gives you a knife, which allows you to score a one hit kill on an enemy without having to sneak up behind them. The new attachments for the SC20K are also very cool. The missions have been made to be open ended, so not only are there multiple ways to approach a situation, but also multiple mission paths to expand upon the experience and add replay value.
The AI is great in Chaos Theory, and the enemy NPC's respond realistically to most situations. They get more aggressive as the game goes on, and really handle themselves well in a firefight.
The co-op mode in the game is great, and really offers some cool gameplay opportunities. The special co-op moves are cool and fun to do, and it almost brings a puzzle solving element into the game at times. It is also very dependant on teamwork.
The multiplayer is basically just an improved version of the multiplayer game in Pandora Tomorrow. It offers new maps and gadgets, and helps balance things out a bit more.
Overall, Chaos Theory is a very complete experience. The single player will take anywheres from 8-12 hours to beat, and then the co-op and multiplayer will easily keep you busy until the sequel. Chaos Theory continues to add innovation to both it's series and it's genre as a whole. If you enjoy stealth action games in any respect, then I strongly suggest that you give Chaos Theory a try.