Here are my first impressions after exploring the northern parts and Telvanni Islands for a couple of days. Using TR1 and TR2 btw, so I have the latest version I think.
First, the exteriors are excellent. They look "natural" but also quite often unique. Nicely detailed. The same goes for the Daedric shrine, Dwemer ruins, caves and Ancestral tombs I've checked out. In short - the quality overall is exceptional. As for quests I've only started/done some short ones so can't really comment upon them, except that they seem to work fine.
Some nitpicks:
Butterflies - I have Butterflies in Vvardenfell as well (from Creatures X?) and they work fine. The Butterflies in TR are quite odd however. They either fly in slow motion - or not at all (i.e. just hangs in the air "freezed").
Adamintium Dagger - I've never liked Adamintium so I should perhaps not comment on it, but for one thing it looks too large for a dagger IMO. Anyway, it clips through the fingers of my char (Better Bodies).
Daedric Naginata - Looks nice plus it was a cool twist to get it. I have a weird minor issue with it, though, namely that the blade (a part of it anyway) flicker. No idea what it might be.
Fort in Bal Oyra - (I think it was there anyway...) I have jaggies on the wall (down the middle).
Outside Ranyon-Ruhn - I have an ugly texture seam.
Note how small the nitpicks are.
Anyway, it's very fun just to adventure around so far. I'm sort of amazed by how this amount of quantity can have such high quality overall. Many thanks to all involved in this gargantuan project.