Everyone I'm sure has some success stories as well as some horror stories.
Personally I've had excellent results from the upgrade as well as a couple subsequent clean installs, but.....
I'm currently working on one for a friend right now that is caught in a loop, he had some problems with Windows 10 that annoyed him so he decided to roll his install back to Windows 7 Pro and now the machine is caught in a "reverting your system back", loop. Spins for a minute or two and reboots and the process starts again.
Unfortunately we don't hear to many of the success stories here on the forum. People come here that are having issues.
Not much reason to post and say.... Hey, everything went just fine.
But I'm certain there are hundreds of thousand out there.
Windows 10 is now installed on over 50 million devices worldwide.
Surely there are a couple of those 50 million running just fine.
SOURCE:
http://www.winbeta.org/news/there-are-now-over-50-million-people-using-windows-10-worldwide