On a semi-related note, I just got back from USH to check out Harry Potter. Had never been able to check out either of the lands in Orlando so this was my first time. Wow! All around, amazing amazing job. Had so much fun walking around and exploring it all. And butterbeer is as good as everyone says it is.
I would say that Cars Land has it beat as far as making you feel like you are somewhere else entirely, but USH wins hands down as far as really immersing you in the world they've created -- every character, every experience, piece of food, merch, etc. is pretty exclusive to Harry Potter and the main attraction from the queue to the ride itself is just incredible. I could have easily done without the 3D though. It really did not add much and the glasses only made everything harder to see during the practical effects scenes and kept reminding you it was a ride.
My only major gripe is the sightlines (why this is relevant to the discussion here). Seeing office buildings, show buildings, and other parts of USH while walking through Hogsmeade is definitely an immersion killer. Hopefully those trees get tall enough to cover it all up soon enough!