After having a terrible experience with Lollipop on a Moto X 2014, I was very hesitant to upgrade the Z2 I got to replace it. I really liked the Z2 on 4.4.4 KitKat with the exception of the low notification volume bug.
I upgraded my (unrooted and completely stock) Z2 over the weekend using Sony's PC Companion software. After the upgrade, I used the PC Companion software to perform a "System Repair" which completely wipes all traces of the previous operating system. I wiped and reformatted my SD card prior to the upgrade. This way, I wiped the phone of ALL traces of any previous operating system data and set it up as a new, pristine Lollipop device.
Here are a few observations after a couple of days with the Z2 and a fresh install of Lollipop:
1. The notification volume bug is fixed. I have used the same mp3 files for custom ringtones and notification sounds for several years. They are now as loud on the Z2 as on any other phone and louder than most.
2. The battery life will take several days to settle in, but even on days 2 and 3 I had 45%-50% of my battery left with 3-3.5 hours of screen on time after a 17 hour day. Not as good as I was getting, but better than most phones I have used.
3. The Lollipop experience on the Z2 is MUCH better than it was on the Moto X 2014. None of the jank and lag and stutter I experienced with the Moto. (And yes, I did perform a factory reset on the Moto X 2014 after the upgrade.)
4. I had no issues with call quality. If anything, it is better now than it was on 4.4.4 KitKat.
5. I still don't care for all the ridiculously bright colors in Lollipop, nor do I like every menu and settings screen to be on an all white background. Alas, Lollipop is what it is. If you want the latest features...
I spent a lot of time in Sony's Z2 support forums and on other Z2 message boards before I performed the upgrade. The consensus seemed to be that quite a few people who upgraded from 4.4.4 to 5.0.2 had problems with their Z2 after the upgrade UNLESS they performed the "System Repair" with the PC Companion software and wiped all traces of previous data from their device during the upgrade. I'm not saying it will fix every problem, but it certainly worked for me.
I am much happier with my Z2 after the upgrade...even if I liked the look of KitKat over Lollipop. The Z2 was fast on KitKat...it is faster and smoother on Lollipop.