Ah, you meant Overdose (standalone addon), I was thinking about a full sequel.

It was called painkiller overdose. It was a fan made mod/project that ended up being published. Unfortunatly they seemed to have lost all the things that made painkiller great. By the time you start seeing decent sized waves of enemies the game was over. The bosses were way to easy and the collectable cards are laughable and unbalanced. About the only things going for it was the graphics compared to the orginal and fairly nice looking monsters, which unfortunaly though they had many monsters you see alot of overlap in attacks between them.
Its not a horrible game but it isn't really a good painkiller sequal either.

A bit of a coincidence, OverTheGun is doing a playthrough of Painkiller currently, should give you a good idea of what the game is like. Mind the heavy swearing and the heavy accent (your mileage may vary) if you decide to check out the videos, though.

Painkiller is pure awesome. Think old-school fps (run & gun, somewhat mindless fun, in the same vein as Serious Sam), then add in awesome physics (there's a gun that fires wooden stakes which is some of the most fun I've had firing a gun in a video game), and finally great graphics and you've got a pretty winning package... I'd hella be buying it myself were it not for the fact that Painkiller Black edition already holds a permanent spot on my game shelf.

Indeed it is good. Buy it. NOW!
Beautiful levedesign (every level is completely different), new types of enemies in every level (seriously) - it comprehends mononious shooting by changing everything every level. Awesome! Ha!