The co-op (offline/online) is quite good. I have found that the single player can be a little difficult at first. Although I'm usually an advocate for single player this really is a game that you want to play with people as it is very light in story.
Geez, all I was pointing out was that the reason it was harder in single player was because it wasn't designed for single player. There is no difference between "Co-Op" and "Single Player." The game doesn't even scale the number of enemies and two bosses are basically designed to require at least two players to beat as intended (both are doable solo, but far more difficult then they are intended to be).
You didn't even really contradict me, I said there was no difference between single player and co-op, not that the campaign wasn't playable in co-op.