It's pretty fun, but can be problematic at least on the first time if your friends haven't played it before. It gives you all the 7 main morphs to use right away so it can be kind of overwhelming. One way it could work is if you play through the game first so that you can get completely used to the morphs that you get later on in the game (this way you also don't spoil the enemies for yourself), and tell the others to concentrate on just 3 morphs, like gun, sword and the fist. This way you can counter the few enemies that need some of the other morphs and not have your friends overwhelmed completely (the easiest difficulty doesn't require the other morphs though). But I dunno, this is just a suggestion.
About the coop mode itself, there are 3 stages of difficulty if I remember correctly, with each difficulty having 2 missions. You obviously need to kill the enemies, but you are also competing against the other players for points and you can also steal their heroes (so you can do bigger morphs but they can do only smaller). The missions aren't very long, but they're fairly varied and it's fun to do them again with friends.
I tried it with my brother but he was pretty overwhelmed not just by the amount of morphs, but also that for a beginner it might be hard to see what's going on. He didn't like it. My friend liked it though and we tried the harder difficulties together too and even though he was a beginner he understood pretty well what was happening.
Overall it isn't really as welcoming as something like 3D World, but if your friends are used to action going on it could be pretty fun.
