I upgraded from Windows 7 Ultimate to Windows 10 PRO and I'm sort of content about it.
Obviously it (still) has a few bugs and lags (but it might be my PC's fault?) sometimes. Apparently it can't handle so many tasks as it's predecessors. For example, on my previous Windows version I had no problem installing some software (Office, to say), running a back-up program, listening to music using WMW and browsing the internet at the same time. On windows 10 it'd lag like hell if I do that (I did). Also menus don't open when I click them or Windows Explorer crashes sometimes for no reason apparently.
But, again, it's still newly released and it has a lot of room for improvement. Anyway, i'm sticking to it for a while hoping for updates.