The environment and eerie feeling this game gives you is like no other. This game manages to feel creepy the whole time and the environments and levels really complement the theme of the game.
The AI is superb and a lot better than most other games. After installing the latest patch, the game only crashed 3 times on me back to windows, and this was through the whole course of completing the game, a stat I feel is fairly respectable.
Give this game a try, it is one of the better FPS's out there.