Any difference in day to day performance? They have changed the runtime from Dalvik to ART which is supposed to make a significant difference in the context of resource management.
How different is the UI from stock Kitkat/Jellybean?
I was using ART before the release of Lollipop. Each app takes up more space, but they launch quicker and run smoother (apart from the ones that aren't compatible, there are a few).
The UI is vastly different in my opinion. The app drawer has changed, the keyboard is different and the notification system has been revamped. I really like the notifications, others may not (the opinion on forums is mixed). The menus have all changed, the 'material design' look is much more consistent now.
The overall performance of the phone is marginally better, it was running well before, it seems slightly faster now.
I also sideloaded it on an old Nexus 7 I had lying around. The ART runtime wasn't previously available on the first gen Nexus 7, now it's default. I haven't looked through the developer options, but I'm assuming the Davlik option is no longer there. The Nexus 7's performance goes from the sublime to the ridiculous. One minute it's like butter, the next the whole system grinds to a halt. Clearing the system cache has improved things considerably though. I suppose the change is more vast on the Nexus 7 and it'll take time to iron out the bugs.
Nexus 9 is performing brilliantly with Lollipop, I haven't had any app crashes or noticeable bugs on either the N9 or N5.