Back in the Win10 camp (yes, I know I'm fickle). Two nights ago I made an image backup of my Win8.1 install, upgraded to 10074 (or whatever). Last night I tried all evening to get the latest update to install, but it wouldn't even start downloading.
This morning I tried again, and behold, it downloaded and installed without a hitch. It's been about an hour since install finished, and the SP3 is running cool with no fan noise, etc.
So far so good, keeping fingers crossed.