I'm not sure how to define good when it comes to co-op as I mostly play alone but I have been playing FC5 with a pal from work. It works fine and is good fun. It does only save the main players progress, so if you were the one invited to the game any progress you make will only be saved on your friends game. I don't fell as immersed in FC5 as I was in FC3 & 4 but I don't know if that's because I played those games alone and it feels more like just fooling around playing co-op. Don't know if this helps you or not...
