It's probably the best stealth game I've played; even so, I'd hesitate to recommend a first-time playthrough in 2012.
The gameplay is reasonably robust. All the way through the game, I kept thinking of the sequences with armed enemies Arkham Asylum; after finishing both games, I think the gameplay in Chaos Theory really is best described as a spiritual precursor to these sequences Arkham Asylum. In each game, you have a robust repertoire of gadgets and athletic tricks which you can creatively combine when faced with any given obstacle. In each game, you have some ranged weapons at your disposal, but you're sufficiently outgunned that you'll have use brains rather than brawn to succeed.
Following this comparison, the relative weakness with Chaos Theory is that its efforts to be realistic are often at odds with its efforts to be accessible; Batman, by contrast, is permitted to sacrifice realism for gameplay. When Chaos Theory is realistic, it often becomes tedious; watching Sam slowly crawl through 40 feet of air ducts is positively mind-numbing. On the other end of the spectrum, several accommodating gameplay mechanics are sufficiently unrealistic that they make the game's premise feel false. For example, it doesn't make any sense that Sam can leave someone indefinitely unconscious by putting them in a chokehold for just 2 seconds, or that a bank guard would set off an alarm during my robbery attempt but not bother calling the police or even waking up his sleeping buddies. The game would be less fun if I had to murder everyone and never set off alarms, but the story and setting feel cheapened because of these accommodations.
Further, Chaos Theory has linear levels and cast of generic characters, though the story is very well executed with dramatic sequences in the first mission and the last mission.
Upon release, the multiplayer aspects received some praise. This isn't relevant for me, as XBOX live is no longer supported by this game, and I have no interest in a split screen mission even if there is such an option.
As far as picking it up for the first time in 2012, I would recommend this game only to someone who legitimately enjoys the stealth genre. I don't doubt that this is still one of the best games in that genre, and even one of the better games for the old XBOX. That said, I kind of feel like I wasted my time playing it. I'm not a fan of the genre, and while I liked this game enough to finish it, I kind of regret that I did. By the time it was over, I was definitely getting sick of it, and I would have rather spent all those summer hours going surfing or sailing.