It's an awesome FPS with fairly open ended levels, where action and stealth approaches are viable gameplay options, aprtly because your enemies have great AI. Also, it has funny conversations which make even the goons you face seem like real people, intelligence items to pick up that add depth and hilariousness to the game world, and most importantly it doesn't take itself seriously. Finally, the game and has the best set pieces(only matched by the sequel) you can find in a FPS.
Oh, and it also stars the best female lead character in gaming, ever.