Lollipop is a reasonably large UI upgrade. I had it on my Nexus 9 and I really liked it. Everything was super smooth and the interface feels lighter and has a lot less chrome. On the Nexus 9 it was very fast and responsive. Whether that is do to Lollipop or hardware is unknown, but the end result is a nice quick device. Too bad the OS was let down by the Nexus 9s poor manufacturing quality with massive light bleed, wobbly buttons, etc.
I really want the new OS on my Nexus 7!