I spent about 3 hours this week doing tech support for my mom over the phone because of Windows 10's aggressive upgrading behaviour.
I told her to keep her the best way to keep safe security wise is to make sure the os/browser is up to date, and because Windows 10 keeps asking to upgrade, she went ahead and did so. After upgrading to Windows 10, all network access (wired and wifi) stopped working completely. The worst part of this is that if you can't get internet, you can't even download new drivers to try and debug the problem. We tried a few things, and eventually just did a system restore back to Windows 8, but unfortunately, the system restore didn't work completely and there was more mucking around to try and get the computer into a bootable state. Then, last night, she said the heard the fan making a lot of noise, and looked down to see that Windows had gone ahead and started installing Windows 10 again. Fortunately, wired network access works now, but wifi is still broken. Hopefully Windows will push an update that fixes the wifi access soon.
Also, there seems to be a number of people here commenting very strongly in favour of Microsoft, which I find very surprising. Perhaps people from MS trying to do damage control? Some people have doubted that Windows is doing this automatically, but during the above debacle, my mother actually spent time to make sure that 'Check for updates but let me choose whether to download and install them' was selected. She even sent me a screenshot asking if this would prevent Windows 10 from installing. A few hours later she had Windows 10 again.