I tried to love Painkiller. It seemed like my kind of game: fast-paced with lots of enemies to kill with lots of weapons. Yet it overshoots its goal of recreating classic FPS action and lands in a frustrating realm of repetitious and boring gameplay. How could a game about slaying demons with a stake gun be boring, you ask? It comes down to implementation: it seems to misinterpret the classic formula and opts instead for a checkpoint-based level progression that condemns you to fighting wave after wave of uninspired enemies while locked in a room until the game decides you've done enough. Miss one enemy, and you have to hunt him down and kill him before you can progress. Once you've done this a dozen times, the bareness of the game's formula will become apparent.
I've been told that if I like Doom, I'll love Painkiller. Why this might be true, I have no idea. Doom is based on an essentially open level where you progress by finding keys to open locked doors. Painkiller, on the other hand, is often a shooter-on-rails, where you are guided into a room, forced to fight respawning waves of enemies, then allowed to progress to the next room where you do the same until the end of the level. Painkiller is far more similar to a game like Serious Sam, although unlike Serious Sam you don't get the outrageous weapons or bizarre enemies. Instead you are treated to wave after wave of boring enemies which you have to kill with weapons that, while original, are more frequently baffling than fun to use. You get the arcade-style gameplay without much else.
Frankly, Painkiller is overrated. It really has little in common with the iD Software classics like Doom and Quake, and frankly your $10 could be better spent on any number of more modern FPS titles that not only look better, but are more fun and more satisfying than Painkiller. Better yet, just play the original Quake if you need your classic FPS fix and you won't regret wasting your time and money on Painkiller like I did.