Having played co-op on launch week, I didn't really see many problems. There was a glitch here or there in act 3, but much less than anyone could expect from a game of this size. I never had to reload any saves, and the game never crashed. The only obvious glitches were certain events happening out of order or not happening when they were supposed to. Apart from that, certain class abilities and features didn't always work correctly, but every cRPG ever released has had those, and has them to this day.