Its really good, my girlfriend carries me through it shes really good at it. I must of played this game for a hundred hours with her. Unlocking thr classes can be done with some text file in the game directory by changing certain trackable invisible achievements to completed, if you dont want to grind out the classes that is.
I liked the acid monster thing, forgot what its called. Powerups being stackable is great, and the action is fairly fast paced.