I got a chance to play this game with my friend the day before yesterday. The experience is dramatically different than single player. It opens up all sorts of options to more efficiently finish the levels, if you coordinate your characters' actions and have a strong enough understanding of the levels to capitalize on your extra set of hands.
I like the back-and-forth, and I think the extra resources, properly used, can make the challenges a lot easier. However, multiplayer presents one big obstacle to advancing in the campaign - the fundamental structure of the game encourages socially awkward behavior.
In the simplest possible terms, the levels are too hard. It is virtually impossible to get them on the first try, and clearing them in two or three attempts is unlikely. If you want to beat a level, you have to commit to doing it as many times as it takes, even past the point where it ceases to become fun and starts to seem like an attack on your personal pride.
Which is something you do with a teammate and not necessarily a friend. Maybe I'm too socially timorous, but "join me in this emotional hothouse and maybe after we inevitably lose for the fifth time in a row we can start trying to figure out who to blame" is not a pitch I'm particularly inclined to make. I'd rather just hang out doing something fun.
I suppose you could, technically, decide to play the game casually, without cultivating any sort of driving creative tension, but I'm pretty sure if you did that, you'd never get past certain levels that require precise coordination and split-second timing. That's the curse of being great. You have to, at least at times, act like an asshole, because pushing past your limits isn't comfortable and making people do things that make them uncomfortable is a total asshole move.
So I have to be content with failing at FORCED's multiplayer by playing it as a normal human being who doesn't take games too seriously and not as an obsessed fanatic who will push himself to the very brink of reason to attain perfection.
But then again, there's still single player . . .