Sounds like I've been fortunate, as on single player I haven't had any issues. But when I try to play with my brother, PC for me, Mac for him, the game runs into serious issues with him not being able to log in, getting randomly booted, or when he can get in having some pretty show stopping graphics issues. This seems to be particularly bad in Lower City of BG, so bad that he basically has to log out and I run the party to the next area connected to Lower City, then hope he can log back in when I get out of Lower City.
I haven't seen any responses regarding these issues, has Larian done so? Even if it's just an update saying they are aware of the problem would be nice.