I just completed three successful upgrades to Windows 10, two from Windows 7 (a T420 ThinkPad and a relatively recent HP tower), and one from Windows 8.1 (an underpowered HP mini laptop with touch screen).
The only issue was as needing to start the upgrade from Windows Update. I initially started the T420 upgrade from the “Get Windows 10” applet, but it stalled “Getting things ready” (I waited 30 minutes, before checking with Google and found stalling was not uncommon when started this way).
So far, I’m really liking Windows 10, although I still prefer the Win7 programs menu, and some areas of Settings feel clunky. However, the Windows 10 UX in general beats both Windows 7 and Windows 8.1
Progress marches forward…