I signed up a while back for the upgrade, and finally last night it tried to execute - and failed.
The message I got this morning was that my Nvidia GeForce 965M didn't have the compatible Win10 drivers. So I went to Nvidia and verified that I had the latest Win10 compatible driver. Manually tried to install the Win10 driver again, but got an error message saying it doesn't work with Win 8.1.... so I have it, and it's working, but it's not compatible. WTF?
Catch 22! Can't upgrade windows because 10 doesn't like 8.1's graphic driver. Can't upgrade driver because I don't have Win10.
Also, there was a cute little message under the 'Failed' report with a link to buy a new computer, when I just bought this damn thing.
Finally, I manually checked the normal Windows Update site where you get your patches and there it was. Gave it a click and it flawlessly upgraded my machine to Win10, which took about 30 - 40 minutes or so.
Anyway, I guess the message here is to manually start the process if you have issues, because the automatic upgrade has some very confusing conflicting messaging and bugs.
Also, at the end of the install there was an option to choose either Express Setup or Custom Setup. I'm generally suspicious of any kind of Express installation options, and I'm glad I checked. Microsoft wants to get reports of your browsing history, and of course wants to set defaults to receiving a bunch of information about you that they use and also to their mysterious 3rd Party Partners. I opted out of all of that, and I recommend that everyone else that cares about their privacy does, too. Naturally, they also wanted to install their new browser to the default browser and I changed that to Chrome.
It's running great, and there is a noticeable improvement in performance. I still need to customize a few things, but so far so good! I like it much better than 8.1.