Mine arrived about an hour ago. I'm running through the setup now.
Early impressions: I really like the soft-touch backing. It doesn't quite have the premium feel of the glass-backed Nexus 4, but because that glass is so darn slippery and fragile, I never take it out of its case anyway. This is still nice, and far more utilitarian.
The screen size difference is immediately noticeable, and I think they've enlarged some UI elements. the overall effect is nice, at least so far.
Update with more:
This thing is really, really fast. My Nexus 4 was no slouch, but this phone smokes it.
Almost all the extra screen size is in the vertical, which makes it just as comfortable to use and access for me as the NExus 4. However, I have big hands.
The transparent nav and notification bars feel completely natural. However, the WiFi and mobile signal icons are now always white, and they no longer have the transmission indicator arrows. I'll miss those a lot, as they let me know when I was having network problems. I'm also not a fan of the new settings icon; I liked the old one better.
I'm not sure that I'm a fan of having Google Now to the left. I like Google Now's contextual updates in the notification shade, but I almost never actually open the app. This feels like a bit of a waste of quick-access screen real estate.
Otherwise, the UI changes all seem for the better so far. There are a lot of little ones.
Rolling SMS into hangouts is a nice touch. I almost never use SMS anymore, since I IM almost everyone I know, and having that extra app I almost never opened was annoying. However, they really need to do a better job of integration. I want to be able to integrate SMS into conversations with people who I also take with on gtalk, like iOS, rather than having different conversations based on the medium. I also want my SMS to be forwarded by my phone so that I can open hangouts on my desktop, chromebook, or tablet, and continue SMS conversations through those devices. I use mightytext for that functionality right now, but an integrated solution would be far better.