Microsoft is pushing Windows 10 very hard and I am not buying their push. Several of my customers have had their computers rendered in-operable or partially in-operable. At one point I was actively supporting Microsoft in providing a newer-better OS. Now my customers have had multiple instances of failures in the start menu and computers that act like they have hardware failures after installing the fast paced updates for Windows 10. They have had work in progress halted for hours while Windows 10 is installing and in some cases more time lost reverting back to the previous version.
Enough is Enough
If you are on Windows 7 or Windows 8.1 just stay there for now. If you are buying a new Windows PC you will almost be forced to Windows 10 and is should be OK.
For most people preventing Windows 10 install is kind of hard but there is a tool to help with that. Never10 from Gibson Research. Please feel free to run the Never 10 utility and turn off the Windows 10 upgrade. You can turn it back on again if you desire.
NOTE: I am not alone in having very mixed feelings on this. ARS Technica has a nice summary article on this