Ok just to put a cap on all this FUD.
I upgraded to Windows 10 by choice. Thus far I don't have an issue with it.
I had it ask me to confirm I wanted to keep Firefox as my primary browser, once. Now being that they have a new browser they want people to try out, I am not overly concerned.
System works fine, it was the space of a few minutes following some guides online to kill off the advertising crap. Removal of the start bar "advertising" was a matter of right click uninstall on a couple of apps, and a right click to turn off live tile for the one I wanted to keep.
Stepping through the various privacy settings was a bit annoying but overall not hard, and most were obvious. Nothing there that Apple/Facebook/Google(android)/any other number of companies haven't already grabbed usually without asking until a version or 6 down the path. (That doesn't excuse it of course, but still)
I had to turn off the "one drive" integration etc.
Its fast, and responsive, boots quick, resumes quick, is generally a performance boost I would expect from the next edition.
The changes to the UI are actually fairly mild. Easy to get your head around if you are used to 7. About as difficult as it was from XP to 7.
Gone are most of the gripes that I had regarding Windows 8 and 8.1. (As a system builder I could not in good conscious recommend my organisation move to Windows 8.* as the paradigm change in the UI was too much.)
Honestly my biggest gripe is the lack of true theming with the UI. I absolutely despise the "flat" look that apple championed and everyone seems to be copying.
Everyone seems unhappy with Microsoft, because they are simply beginning to do what Apple, and Google already do.