I will share my experience,
I'm loving the change to 8.1 from 6.3.1, it is much faster and cleaner in look. Now for a fuller disclosure:
this is the first iPhone (a 4s) and I've only had it for a week before I updated to 8.1, aaanndd, it was just reset as new so it was as clean as could be for it's age. I have no issues to report, wait, I did have it "hang" when I updated. It went thru the download and started the install and then the bar stopped for several hours near the end and I though I just created a brick, and no it turns out I didn't. While reading all these posts I read about just (again this was after an unusually long time) shutting everything off, waiting for a short period (few minutes for me) rebooting. The phone said that it was updated and wanted to go thru the process to complete and activate as normal. Most all of my info was there.
That's my 2¢ worth...