Usual rules apply before doing a system upgrade: back it up first, test your backup and then you're good to go.

Core functionality is super-solid. It's a lightweight (relatively speaking), performant OS. New window management and virtual desktop tools are great. Edge is the beginning of something wonderful. It's like Chrome used to be - lightweight and fast. It hasn't got extensions yet, so you might want to hold off on it as a daily driver, but press F12 and marvellous things happen. Yeah, the dev tools are pretty cool.

I found the upgrades (in-place from 8.1) on my XPS 13 and desktop to be absolutely painless. Didn't lose a thing.

I'm led to believe (haven't done it myself) that the rollback works well, too.

TBH if you've got a backup, get it installed. Stop faffing and procrastinating and reading tales of woe on the internet, and JFDI.