I've completed my first playthrough and was really unimpressed with the final act. The game had a lot of variety up until that point - both in terms of narrative and level themes - but once you hit ActIV it's just battle after battle, with very little to keep you interested. It just feels like it was rushed and it makes the game feel short. I didn't get the same feeling with Dragon Age, by comparison.
Still, the game is a lot of fun co-op. I'll be starting my second playthrough on Nightmare, mainly so I can upgrade my jeweller and blacksmith as they both require training pages and they're exceptionally rare in Normal difficulty (I didn't come across any). I like that upgrading them is done on the account rather than the character, so progress is maintained. I've got two characters - demon hunter and witch doctor - and I found them both to be very competent, though the demon hunter is my favourite.