I updated my SP3 W8.1 to W10 on the first day it was available. I was very satisfied... initially. I think W10 does a much better job than did W8.1 of separating desktop and tablet modes and being explicit about it. However, there are also some things W8.1 did better than what W10 does. For instance, if I am in tablet mode with rotation lock on and I put the SP3 to sleep, and then attach the keyboard and wake up the SP3, I get asked if I want to go to desktop mode and I click yes. Now I cannot adjust the rotation lock in any way, I have to detach the keyboard, and switch to tablet mode, only then can I unlock the rotation lock. This W8.1 handled much better.
Also, after the cumulative update, the built-in mail app no longer works with my company exchange server. It was a bit tricky to get it fully functional but eventually I managed (had to set up the account using the calendar app), but after the cumulative update the app is just a dust collector.
In short... the honeymoon with W10 on my SP3 was short. But then, I was never happy with W8.1 either, so that divorce was still a good thing.