Absolutely incredible.
The review here on GameSpot discusses the story, saying that it is weak and not as good as the original. Sure, it's not as complicated or deep as the original but it's still very captivating and emotional... I'm loving every minute of it.
Other than the story and the atmosphere of the game, the best thing about it is the gameplay - which 2K have got spot on. The new 'dual' weapons which involve one hand controlling plasmids and the other controlling a weapon may seem a bit pointless, but when you really get into the game and fighting huge amounts of splicers all at once, you'll understand the necessity.
If you want to play a game that is immersive, creepy, fun and damn interesting then get this game. If you don't, then I'd suggest selling your 360 because you're not making the most out of it.