Decent shooting engine.
For a simple time-waster, this is the best you'll get. The game engine is extremely sophisticated and sensative. Your moves mimic your real-time reflexes. No lag, bug glitches, or anything that would hamper lifelike gameplay. Headshots are cut clean. The game is difficult, but not impossible. (beat it after 3 tries) One might complain of lame graphics, but in case you didn't notice, this is a stick-figure game. No one gives a fuck about graphics. They are sacrificed for detailed gameplay. And this game does an exemplary job of it.
My only complaints are the shitty ending and lack of decent health recharges.