Not too impressed really.
Suddenly everything is bright. Earlier it used to be dark. A dark skin option was needed, honestly.
The old Gallery is a better app than Photos.
The new Notification system is marvelous though. But but but, swiping upwards BEFORE you can enter your unlock pattern is irritating.
I can understand you having to swipe first if you have notifcations on lock screen turned on, but if notifications are off, the pattern panel should appear directly by default. Really irritating this.
Google had purchased QuickOffice a couple of years back. The new apps - Docs, Slides and Sheets are good. But PDF reader is slower than before.
Material design looks cartoony to me, but I'd say the usability in all the core apps has increased with the new interface.
imo, Lollipop is more about updates to the core apps. Otherwise it is bland for me. I preferred the look and feel of Kitkat.