A mix between Horror and Shoot em up, which ends up very good!
Well you can pretty much say that painkiller has an old concept which is "complete this level and move on to a harder level" What makes the levels harder are not only that more gore will be splattering around you. even the levels will be harder to complete as the paths to the end of the levels are harder to reach after each chapter.
I myself like when you really can feel that you have killed something in a game by seeing the blood splatter around, its a very good way to get rid of your aggressions - watching something die, or even better a lot of things dying.
The story is pretty lousy, the game is all about killing demons, and that is what makes the game great too!
My experience is that the graphics where extremely good untill Half Life 2 released, but even now the graphics are a joy to look at, even though the graphics today are more detailed than both HL2 and Painkiller.
As for the sound, they could have made it better, like added more horrifying death-screams, but in overall, the sound blended in to the environment. The controls are great too, as for the awesome weapons which the game contains.
So i easy words, a great game for you guys who need to get rid of your aggressions, also a great game for you guys who are fps-fans, as the ammo can be very limited on some levels, which makes the tension very high and keeps you playing untill the end of the game.