Fun for 15 minutes, followed by boredom.
Painkiller has been praised to high heaven and back as a return to old-school FPS style. I'm convinced that the reviewers claiming this have either forgotten old-school shooters or never played them in the first place.
About the only common factors Painkiller shares with the likes of Doom or Duke Nukem is that a) it is a first person shooter and b) it features an excuse plot. It's not so much an FPS as it is Smash TV
in first-person perspective. You kill a wave of enemies, a door unlocks, and you proceed to the next room. The catharsis factor wears off pretty quickly and it gets monotonous and tedious pretty quickly. Which wouldn't be a problem if this was an arcade game played a quarter at a time that lasts all of half an hour; but this is a full game. The enemies have no AI to speak of. They won't try to dodge shots like the Skaarj in Unreal or gang up on you like Pig Cops in Duke Nukem 3D.
The level design is basically non-existent. The levels can be described as "interconnected rooms with a theme". The themes don't really continue from level to level, and even within the levels there isn't a whole lot of sense to be made. Why are you fighting Ninjas in an Opera House? Who knows. It seems like the character modelers and the level designers just made whatever they wanted with whatever theme they wanted and haphazardly slapped things together.
Music is generic thrash metal power chords. Nothing notable.
The weapon selection is pretty uninspired, and none of the weapons really feel very powerful, except for the stake launcher. Your character also moves pretty sluggishly, for a supposed old-school shooter. And honestly, Painkiller doesn't resemble old school shooters at all. As far back as Wolfenstein 3D you had to navigate mazes, hunting down keys and searching for the way out. There are no monster closets to surprise you, just spawn points that cough up enemies till they're dead, and it's time to move onto the next one.
I found Painkiller to be an absolute chore. I just gave up after the 2nd boss, because I honestly wasn't having fun.