I haven't tried the multiplayer support yet (I'm guessing it's no where near as nice as say Steam games), but the single player story is amazing. It is also one of the freakiest FPSers you will ever play (though I can't decide whether it is scarrier than Doom 3 or not)
Beyond the story, the artificial intelligence is great. From footsteps to flashlights to peeking out of the shaddows, one wrong move will find you in the middle of a massive firefight!
Enjoy


