Playing exclusively in co-op, it's a bug fest.

A major character was invisible all through his introductory cut scene. Random voice acting cuts out. Both players can have cut scenes going at once, but only one will get voices.

Some of this has not been thought through properly at all. IMO, important cut scenes should automatically play full screen for both of you. When you sleep, sometimes there is a cut scene that's very similar for both of you, playing on both halves of the screen. There's no way to full screen this, and one player isn't going to get voices again. This would have been better as a single cut scene featuring both players.

Performance is fairly poor as well with large (2-3 seconds) pauses when swapping characters, or summoning things. It seemed to start doing this after 15 hours or so.

The menus are clunky as all hell. Want to know what that mysterious debuff does? Good luck with that. R2, up, X, L1, L1, down, down... oh it's faded off.

Edit: Also, your username is overlaid at the top left of the screen at all times, right where all your roll info and character likes and dislikes appear, making it hard to see them. The tutorial also didn't work, but that seems to have been fixed with the patch released today.

Edit: As of the 22rd September patch, the performance is massively improved.