When I first played this game it reminded me of the first time I played Doom. The game is dark and twisted giving it a feel that is hard to describe, and harder to capture. In Doom, it was fun just to go hacking up bodies with a chainsaw... In Painkiller you get a similar feeling using a stakegun. Its fun to shoot wooden stakes and see your enemies get impaled against a wall.
I do not play many FPS games, but Painkiller is one of the few that I enjoy.