KitKat to Lollipop on the Sony Xperia Z1 Compact.
Best to let someone else break their phone first. I always prefer not to rush and upgrade as soon as one arrives, I would rather wait a little and see how things settle down.
The only issue I have seen with other devices (not Sony) is that some are having battery issues and some WiFi.
UPDATE (23rd December 2015)
I thought it was about time I left an update. I did upgrade about a month later and have been very happy with Lollipop. There is now talk of Marshmallow coming soon to newer Sony Xperia model.