People were expecting too much from Resistance 2. It's a very good shooter, and a very good game. The campaign is mediocre, the competitive is pretty good, but the co-op is the star of the show.
It's definitely worth a purchase. KZ2 doesn't come out for a few weeks, but if you have to choose one or the other, I think the decision is obvious.
KZ2 FTW! (Sorry off topic )