MDK 2 is a good 3rd person game, let's get that right out front, but it's suffering from a clear case of sequelitis.
The developers took a lot of the formula of MDK, the huge set pieces, the fun music, the goofy enemies, but decided that wasn't enough. So instead of simply improving the formula, they added two characters, the professor and his dog, which ultimately don't add anything real to the game.
The Dog's missions are extremely scaled down versions of MDK, pure shooting, without the fun puzzles or the sniping. The professor's levels accentuate the puzzles, but blow it on the combat. Why seperate the game elements that worked so well in the original? Why can't I just keep playing as Kurt the janitor and reluctant savior of the world? Because the developers thought they needed to have some extra blurbs on the back of the box when they sold this one out.
As I said, it's not a BAD game, it's just not the 3rd person romp that the original MDK was. If you loved MDK and want more, you could definitely do worse, but my recommendation is try Giants instead.