I was very surprised last Wednesday (29th) when my MS Surface Pro suddenly started to download the upgrade to WIN 10 and start installation. The whole process took no more than 20 minutes, was totally painless and everything runs perfectly. Win

Next, wait for the upgrade to become available for my desktop system - Acer one-piece with touch screen and wireless keyboard and mouse. Yesterday morning it became available via the little windows icon, so I started the process after having had such a wonderful experience with the Surface. However, things were VERY different! Like chalk and cheese! The download took almost 1 hour, then the upgrade process started... ... ... ... After more than 2 hours it had reached 74% and appeared to be making no more progress. I had to go to work, so I decided to leave it doing what it was doing and see what the result would be when I got home. Eight hours later I came home from work and it said it was at 99%. But... my wife said it had been showing 99% for the last 4 hours! So I thought I'd nudge the mouse, or press a key as these things sometimes give a little shove! Nothing! After waiting another 20 minutes I decided the only way out was to do a hard shut-down by holding the power button and then restarting a few minutes later, hoping that, at best, it would restart back in the good old WIN 8.1 (Pro, by the way). To mys surprise it started up showing that it was finalising the upgrade, and then went to the log-in screen. Joy! ... ... ... However! Not all my app tiles were present but, worse than that literally NOTHING worked! Except, luckily, the start menu. I experimented for some hours to see if I could make any tweaks to solve the problem but to no avail.
I used the working functionality of the start menu to ROLL BACK!

The upgrade also became available on my wife's laptop (Packard Bell running WIN 7 Home). Started the process. It downloaded the upgrade and then asked to restart in order to prepare Windows for the upgrade. When selected we got the usual display of "Preparing Windows" 5%, 10%, 17%, 35%, ...75% ... 75% ... 75% ... ... ... 75% ... ... ... ... 75% ... and just never seemed to go beyond 75% and after nearly another hour stuck at 75% we just gave up! Shut everything down and went to bed!

To be honest, not in a hurry to dip the toes into that VERY cold water again!