I had two computers come up running and they have been running for 7 weeks now (with only W10 annoyances, no disasters). A third computer spanned about 100 hours of my time (including research and "waiting for W10 install to crash" time) and took 9 attempts to get the system up and running (and it has run satisfactorily for about 4 weeks). Several times it tried to revert to my W7 system but failed miserably. Even when it did revert, the system looked nothing like what it looked like before I tried the update. I still keep the W7 image around for all 3 computers "just in case".

Obviously, the horse is already out of the barn for these folks but anyone reading this thread that hasn't upgraded yet would be very wise to make a full image backup of his system, on external media, before performing the upgrade. (Be sure you understand how to do that, including building a boot disk so you can restore the image should the need arise.)
Plus plenty of tutorials to read here not only this one which goes over that stuff, how to prepare.30 days to roll back rule after upgrade.