Personally, I wasn't amazed at the single player campaign. The fact that you can do it in CO-OP makes it a lot more fun, but the storyline is still pretty 'meh' if you ask me, and the missions seem a bit repetitive.
Multiplayer on the other hand, is loads of fun. Every game is different, and even though there are only very few MP maps out at the moment, it's still very enjoyable.
You, Duality and me should have a game sometime ;)