I've beaten each campaign twice-- once on my own, and once with a good friend of mine. I definitely had more fun playing with him (Aside from the fourth scenario, as that one was very clearly designed to only be played by one person), but it was still a fun single player game. At the very least, it wasn't nearly as bad as Resident Evil 5, where your AI partner was dumb as rocks and died constantly.
Now Playing - Resident Evil 6 (360), Kingdom Hearts 3D (3DS)
Yes it is a fun single player game, in fact in single player mode you can go your own pace which makes it more enjoyable.
FYI, this game is so "terrible" that many seem to spend 24 hours here complaining about it even 4 months later.
The game isnt perfect but it sure is fun. The most glaring problems are the story isnt great, you cant shoot some of the enemies until they're standing or completely spawned, no weapon upgrades outside of skills and you cant customize your weapon loads each character is static with their own set of weapons many of which are the same.