Fantastic firefights with poor level design.
The enemy AI is also spectacular. The enemies on harder difficulty behave just like human opponents in a net deathmatch. They will run away, dodge, ambush you, flank and yell out tactics when you flank or charge them. They actually feel like intelligent beings.
The particle effect system is absolutely spectacular and the weapon models are gorgeous. All these physics do take a toll on your CPU so plan on playing with only the best on the market. I have a fairly beefy Athalon 3500+, 2 GB 3400 dual-channel RAM, Geforce 6800 Ultra 256mb and 2 (raid 0 setup) WD Raptor hard drives and I had to turn down some of the details and still the game had slight hick-ups in certain levels. So that should give you an estimate of what to expect as far as computer requirments go.
The sound is excellent on a surround sound system with you being able to distinctly hear every shell drop to the floor and ever creepy scream and whisper in the game.
The game has a great creepiness factor (i was more creeped out by Doom 3 however and System Shock 2 doesn't even belong in the comparison).
The absolute weakest part of the game is the level design. To summarize the game levels I just have to say 3 words; office building, old basement, doom-like lab. That's about as much variety as you're gonna get. It would be ok but the office levels are simply horribly recycled. Everything looks the same with a slight twist here and there. The game is also fairly short.
If it wasn't for the mind-blowing gun fights, this would be a very bland and average FPS since there is also no innovation in the game whatsoever. If you're looking for a more concise story, I would go with Half Life 2.
Unfortunately sometimes while playing the game, I wanted to summarize it as "A wild, particle filled romp through an office building."