The Lollipop update from Google has been far from smooth. I have a Nexus 7 2013 and the first two rounds of updates didn't make it out of the "soak test" phase before being pulled. I got the update on the third release cycle and it seems to be OK. Battery life seems about the same and performance for most apps is still OK. However, depending on the number of apps you have on your phone, the update is NOT fast. All the apps have to be recompiled for ART....I think that is where some of the problems occurred.
Personally, for my Z3, I'm fine with KitKat and if it takes Sony a little longer to get the upgrade right....I'm definitely OK! I'm in no hurry. FYI, battery life should actually be better with Lollipop, but, like others, I'm skeptical.