I have built my own desktops since WIN95, usually installing new Windows versions for each build. None of these were "Top of the Line" builds, although they were all built for flightsim use. When WIN10 was announced as a free update, I was using WIN7.5, 64bit. As I was getting ready to start purchasing components for my next build, I decided to go ahead and try it. Expecting trouble, I backed everything up to an external HDD. I was actually following along with the threads here on SOH at the time about others info on WIN10, especially Texnetcop's info. (did I spell that right?) As it turned out, the update went nearly flawlessly, no loss of info or programs, except those that would not work under WIN10, like others have mentioned. They were actually saved in a separate folder by WIN10, in case I wanted to switch back. My WIN7.5 was saved in another folder. Never went back. I still have that old tower in the closet, and have used it a couple times for odd things. Still works fine. All of my flightsim programs worked fine.
My present version is WIN10 PRO, purchased 3 builds ago. As WIN10 CAN BE moved to new builds without repurchasing, unlike as required for older WIN programs, I have had no real problems that weren't my own fault. I have FS9, (5 versions, including the "Golden Wings" and "Silver Wings" versions,) FSX, and P3Dv1, v2, v3, and v4.5, all working fine. Each major version is on it's own HDD, with the P3Dv3 & P3Dv4xx being on a 238G SSD, with the Simobjects, and add-on folders being junction linked to a separate 1TB HDD, and all the scenery on 2x3TB HDDs. I have never had any problems with any of them, other than when ORBX changed from the older "FTX Central" to their present "ORBX Central." THAT was my biggest headache!! I was actually able to transfer my WIN10 Pro license to this latest build even though I changed hard drives, from HDD to SSD. A short phone call was all it took, explaining what I had done. The only thing I was told is that I would have to purchase another license if I was to fire up the old tower with the original HDD.
The auto update features are kept turned on, restarting when I am ready to shut down for the night, or immediately if not busy. I also keep the UAC turned OFF. I do have Norton Internet Security package(now 360) as well as "Malwarebytes" installed, and have never had any problems. (Yes, I have had intrusion attempts, always stopped by one or the other.)
I am NOT saying any one else hasn't had problems, but as many have mentioned, I myself have had no problems that can be blamed directly on WIN10. The interface is a little different, (actually a lot) and it takes some getting used to, but once learned, easy to work with. I find the "free update" version easier to get along with than my wife's pre-installed HP-OEM on her HP notebook.
I also have a Toshiba "Satellite" notebook that came with WIN7 that I updated to WIN10 HOME way back when, and it has been pretty much trouble free since. In fact, it just recently updated to build 1903, and so far, I like it. (This tower on WIN10 PRO is still build 1809.)
Just my .05 cents.