Obviously a lot of things missing, but it seems stable enough as is. There is one usability bug though, in that I cannot drag spells onto action bar slots that were previously blank. I can drag to replace a spell with another. Tested with only nUI6 active and then again without any addons.
Works okay for me cause I disabled addons, added the new buttons, then re-enabled -but naturally this is not ideal.
Another facet missing is the cast-bar, but I think that's just not done yet rather than bugging. Also missing are the Rune-bar for DKs, and the nui Aura handling. Bags are also not handled, which means they appear below the viewport and are unusable (along with no minimap button bag). That's what I've noticed so far - I do like the new touches on the look though!
No complaints, just listing them so people know what's happening
EDIT: Just noticed that the Pet action bars behave properly (allowing skills to be dragged onto them and etc.)