The upgrade went fine, but, once at the Windows 10 desktop, nothing worked. No apps launched, could not access start menu, right click on desktop and shortcuts did not work, no hover text on any tray icons, etc.. The only way out was to hold down the power button.
Before you ask, specs are fine (quad-core i7, 16GB RAM, 512GB and 256 GB SSDs, etc.) I was right at the limit on available disk space for the upgrade on my C: drive. I'll move all my user public folders (47 GB on a 100 GB partition) to another drive and create a directory junction in its place and try again (I restored my Win 7 image to the laptop). Anything other than low disk space that is known to cause this? I have found nothing via Google about this particular issue when upgrading and the low disk space is just a guess on my part.