My HK-sourced, recently-rooted international phone here in NZ started offering to update while I was travelling last week and I pushed the go button on my return on Friday. Homescreens were lost and required re-establishing, but all else pretty much in place. The UI improvements are nice and almost everything works just as it did before, or better. But the negatives I've seen are:
a) the News and Weather app I really like disappeared - I got it back via a sideload of GenieWidget.apk obtained by searching xda
b) for some reason, the /mnt/sdcard/external_sd logical path was renamed, breaking my nice MyBackupPro and SugarSync cloud backup combo -- only half back together at this stage (SugarSync's UI gets less intuitive with every new release)
c) Definitely some blank-screen and reliability issues that were not there before - usually waiting them out works, but not always. Seen with key apps like Browser, etc.
d) My worst dislike is that SMS notifications now no longer contain the text of the SMS, just unhelpful "New message" text - I have to click and go into the SMS app to know who it is from or what it was about. ICS was way better in this respect.
e) Had to uninstall the various non-useful german apps it came with, just like the update last August
e) No confidence in the S-Voice and assistant stuff -- very limited and dumb in my early tests, disabled
f) No noticeable change in battery life or memory, but I have had some apps frozen for ages and just froze some more
My note is still one of the best things of any kind that I've ever bought but is now a touch less good overall due to (c) and (d) -- hope some good updates come along to kick it back to my earlier high level of appreciation.