I was very hesitant to upgrade my Z2 to Lollipop. I eventually made the decision to go ahead and take it on...for better or for worse. My experience with it is pretty well documented in this thread:
http://forums.androidcentral.com/so...-ringtone-volumes-fixed-lollipop-upgrade.html
If you do upgrade, I HIGHLY recommend you do the System Repair after/as a part of the upgrade. That way you are starting with a completely clean install of Lollipop. See my comments in the above linked post for details.
As far as battery life goes, mine is, if anything, better on Lollipop. I usually take my phone off the charger at 4:15am and put it back on the charger at 11:00pm, which is nearly 19 hours off the charger every day. My normal use runs about 3.5-4 hours of screen on time and it usually has between 50% - 55% of the battery remaining. The heaviest use day I remember was just under 7 hours of screen on time and it still had 25% of the battery remaining when I put it on the charger that night.
With the Z2, I don't worry about battery life or if it will get me through a day. Ever. It is awesome to have an Android phone and feel like there is no way I could completely drain the battery in one day's use.