"Since the song order is based on release history rather than difficulty for once, the progression curve is wonky as fuck. In one song it's monotonously bouncing your thumb up the strummer like a masturbating stroke victim and the next it's rattling the buttons like you're trying to disentangle your hand from the udders of an excitable cow."
(..)"And die you will. A lot. At least at first. Overdose is not terribly well balanced. The very first level starts out simple enough with some brain dead zombie things. But moments later, your woefully under equipped character will be attacked by enemies baring shields which shrug off nearly every attack unless you time it just right. By the end of the first level, the game throws an absurd number of monsters at you. If you manage to survive that, youíll find that the difficulty swings to and fro almost randomly. Sometimes youíll get a level thatís a cakewalk followed by one thatís punishing. Compounding the difficulty issue is the uncanny way enemies appear to spawn into the level right on top of you and the way your shots (especially with the rocket launcher) will sometimes go right through the bad guys without touching them."
Gametrailers' review of Painkiller: Overdose