The best game ever created to date, imho. It isn't graphically spectacular by todays standards, but that really doesn't matter as you become completely immersed in the game's world. Sinister conspiracy theories confront you at every turn, and there are points where you don't know what to believe.
The game is an excellent merger of FPS and RPG which hadn't been done before, and still hasn't been bettered since. There is the chance to choose many ways of approaching most situations within the game.
The fact that I've replayed this game every year since I got it (um, 4 years ago I think) speaks for itself - truly outstanding