You like slow tactical shooters were you have to think before you do anything? You like games like ARMA but want a little, very little, more action? Than this could be your game. It's old, so are the graphics and everything else, but the gameplay is really good. The game is challenging even if you play on "normal".
But the real fun is the coop. Buy it only if you have some friends who already have it, or also are ready to buy it. The coop, with up to 12 players, are a very realistic experience. I play the game with a friend who is with the military and he's very pleased. The only thing really negative about the game is that there is no bullet physic.
To sum it up: Very good tactical first person shooter with coop.