F3 Co-op is by far, THE BEST, and MOST entertaining thing I've played since.....ever.
It's so freaking awesome, that i decided NOT to play campaign anymore. Well, at least until I get bored of Co-op and return, but until then, everyone should give it a try. It's...spectacular.
I do wish they would fix the weapon levels not saving though, kind of annoying that I get the U-100 to level four with a buddy and re-launch the game later only to have that hard work erased.
I enjoy the heck out of Co-op aswell. Even with random people. I've hadn't had so much fun with Co-op since Left 4 Dead.
I think it's great aswell the weapon unlocks are shared between Multiplayer and Co-op. Gives you a bit more progression. And it helps you if you aren't a champion in playing vs other players
--------------------------------------------------------------------------- ~Infinity is a part of me now~