Well I'm sure it can be fun if you explore the entire map with friends. I've played the beta alone and while it's quite fun to give the order to take down 2 or 3 enemies at the same time, I feel like the idea of having to go to a precise location to get the weapon part that I want might be a huge waste of time in a co-op session, at least with randoms.
I've heard stories during the beta about people shooting and tea-bagging their team-mates instead of helping them, so I'm a bit worried about the co-op.
Like I said in my previous post, I really want to like this game. One of the best co-op experiences I've had was on Far Cry 3.
Unless you RP the game, I don't think it's "fun' to travel a long distance between a point A to B in a yuge and empty map.