I'm sticking with 7 for a good few reasons; I did have a drive with Win 8.1 installed on it when I bought my system - Alienware 17 laptop which I chose for the (semi) portability of it and the hardware which is pretty adequate for most needs including FSX, and latterly P3D. When at home it is attached to a 30" Dell monitor and a number of FS-related controllers. I just couldn't get on with Windows 8.1, it simply 'didn't suit' and as I had bought 2x 2.5 SSDs and an mSATA SSD and had a licence for Win 7 64 Bit, the Windows 8 drive was popped into a drawer with the intention of popping it into an enclosure and using it as external storage.
In the event, this didn't happen as my wife bought me a USB 3 External drive as a present so the drive has sat idle for a few months until W10 poked its spotty snout above the parapet. I switched out the primary drive and installed W10 as an upgrade from 8.1, but decided after a bit of faffing about and trying to install some legacy programs that W10 will not play nicely with, I couldn't be bothered, and since then I've done everything required to prevent a crafty 'upgrade' to 10 from happening; it became a war of attrition and I've used the GWX control panel, as well as a couple of other tools, registry fixes and so forth; if anything my resolve has been strengthened against W10 by the continual reissues of KB3035583 and its friends, all components of the forced install of W10.
So to sum up my reasons for not going over to W10 are: preferring the Win7 interface, keeping the ability to choose which updates to install, a dislike of MS use of telemetrics, spyware, targeted advertising, invasion of privacy...... the list is endless.