By far, the best FPS I've EVER played!
Only thing I could criticize is that the environments get a little repetitive, it feels like the designers were pressured to make the game long, and they put a lot of needless twists and turns into the levels so gamers wouldn't complain about the game being too short. But it's a minor gripe.
(Why do they make reviews a minimum of 100 words? Oh yeah, I work at Gamespot, I'm going to go yell at someone for it, but for now I think I'll use this nonsense to fill the space.)