Painkiller is pretty much like Doom with a facelift...and that's a good thing. A very good thing.
Painkiller is a very simple shooter that is a lot like the shooters of old. You play as a guy who died many years ago and has been stuck in Purgatory. At the start of the game, an angel tells you that God is giving you a chance to get into Heaven. All you have to do is kill Satan's four general.
The game takes you to a bunch of different locales somewhere between Purgatory and Hell where you'll fight all sorts of Lucifer's minions with four different weapons. Each weapon has a primary fire and a secondary one which usually shoots something completely different. The best gun in Painkiller and probably in any game anywhere is the combo Rocket Launcher and Chaingun. It packs a hell of a punch and seldom runs out of ammo.
Like I said before, Painkiller is very straightforward. You walk into an area, the doors in front of and behind you close and tons of enemies run in. Kill the enemies and the doors open. Proceed through the door. Repeat.
That's pretty much how the whole game plays out and although that probably sounds pretty boring, the variety in enemies and locales keeps the gameplay pretty interesting.
The greatest thing about Painkiller is the boss battles. All of the bosses are gigantic - literally stories high and they all wield some sort of really deadly weapon. That smashes your surroundings to bits (the boss battles are where the physics engine really shines). When you kill a boss, you really feel like you've just done something amazing.
Painkiller is pretty much just a really fun, stupid, straightforward game with hardly any plot and absolutely no depth to it. It can be most easily described as a pretty version of Doom. But hey - is that a bad thing?