I had problems the first time I upgraded and rolled back to Win7. I have upgraded again more recently and am currently very happy with it.
Two other computers (a laptop and a PC) in my household have also been upgraded without problems.
If I got to Win10, then want to roll back to Win7, or dual-boot, how do I do it?
Rolling back is straightforward for the first month after upgrade. Go to Start, Settings, Update & security then select Recovery. You should then see an option to go back to Windows 7.
Dual boot is more complicated. You can find some instructions here.
now won't even load the settings page so I can go back
If you want to go back I suggest you search for "Recovery". That should allow you to go straight to the Recovery page without going through Settings. You may also want to try creating Windows 10 media (bootable DVD or flash drive) and doing a fresh install. A number of people report that this resolves performance issues.