Good but not great.
Secondly, this game lets you play as the bad guys. Which I thought was rather innovative; not many FPS let you do that at the time. Lastly, the multi-player is a blast! Now anymore there aren't many players online but if you are able to gather up a group of players you'll find yourself playing for several hours.
Now the worst part of the game is the AI in the single player missions. It is absolutely awful. Most of the time the AI bots arm themselves with pistols or just stand in one place getting blown away without ever reacting.
Overall this one of the better FPS games out there and one of the least widely known.