If you've played painkiller (or only the demo), imagine this:

Take the game, add 2 ok weapons, a few enemies and one kickass visual effect.

Then, make levels that aren't half as cool as the old ones.

Sure, their visual style still rocks, most of the gameplay still rocks (if you like painkiller), and the bosses still rock. However, the developers decided to put jump and run passages in. I've hated first person jump and run ever since half life. In the coliseum level, there's like an hour of it where you walk on a thin wooden beam and there's stuff spinning around that you have to dodge. It's not fun at all, just REALLY frustrating.
Also, there's a buttload of scenes where you're in a fixed ride and have to shoot stuff. A prime example is the rollercoaster ride in the circus level. Sure, it's cool, but you have no effect on whether stuff hits you or not. And you can't kill everything before it fires the first shot.

Still, it's a pretty fun expansion. If you liked the original, its worth the 15 bucks.