Painkiller is a plain and simple masterpiece, and the last of a seemingly dying breed of just plain fun FPS.
The important part of this game is shooting demons with awesome weapons, including the ever so fun painkiller. Simple weapon, it's like a giant fan rotating blade you mow down demons with.
Then there the super fun stake-gun! It's just a gun that shoots stakes. It's an awesome physix based gun. You actually get to pin demons on walls. Nothing more fun.
The ninja star/shuriken gun is brilliant fun as well. That's what this game is: awesome fun and shooting enemies with creative guns! What more do you need? Really, what more do you need? Realism? A story? Pff, those are overrated!