Aloha,
I intentionally downloaded the upgrade, and really have not had any serious issues...but a couple of minor annoyances.
As mentioned, this upgrade is in two downloads. The first went smoothly, took about five minutes. I then checked for updates again, and started the second download. I had noticed during the first download, a message near the end, "Optimizing" 224 applications (the actual amount I had installed, more or less). During the second download, similar message, but with over 350 applications. Based on this, the first thing I did once installation was complete, was to power down and restart, then once it had fully booted, I immediately ran CCleaner, and then All in One toolbox. Again I powered down and restarted. At this time, with wi-fi on, after a few minutes a notification came up regarding Google Play needing an app updated. I allowed this, and it actually took quite a while, probably almost 10 minutes of downloading, before I could "install". The install happened quickly, and all seemed well. A little later in the evening, a handful of apps were automatically updated, after which I again ran CCleaner and All in One, then restarted, and have had no issues since. I probably spent about 40 - 45 minutes total to complete and cleanup the upgrade. AnTuTu score is 41083. So far have not noticed any unusual battery drains, although I will make the comment that I did learn with 4.4 that if you leave wi-fi on, and it connects to a weak source, downloads will take forever and battery consumption goes up. Better to turn off wi-fi and download with the 4G LTE. This also speeds up browsing, when wi-fi is trying to work over a slow or intermittent hotspot.
Annoyances with upgrade...yes, a couple, but I'll get over it. I don't care for the white background that is the "new" default, apparently with many apps. Can't seem to change this. Notification actions have changed on some apps, and again, cannot seem to change back to previous behavior. Specifically, GSam Battery Monitor, previously I could only have icon on notification bar showing % battery remaining only. Now, it insists on always showing a full notification on lock screen at all times (when running) as well. This hides the date on the lock screen. Also, I have double notifications for Maildroid now. I have not tried to figure this one out yet. I do also agree with another poster on the reduced size of the phone "keypad", definitely NOT an upgrade!
On the PLUS side of things, it seems the "Task Manager" now actually closes all previously opened apps.
I have not had any issues accessing previously saved camera images or music files on my sd card, and still can save files to the sd without any problem. No problem making or receiving calls, including VoLTE.
All in all, not a big deal one way or the other. Of course, if you are the type that has to try all the most obscure apps, and never perform even basic maintenance of the device; or drop it, drench it, and otherwise abuse it, YMMV.