Recently upgraded to windows 10 from 8.1, very few problems, I've got it on my laptop as I was an insider but I've finally upgraded my main desktop.
Firstly, Never received the notification as such, I just kept clicking on the 'Get Windows 10' app to see if it would let me but it only ever gave the option of, 'Okay, notify me when ready' except! for yesterday when it gave me the option of 'Upgrade anyway (not recommended), I accidentally closed the app before clicking on the upgrade button and when I went to reopen the app, the upgrade button disappeared!
So I opened up Windows desktop update and there it was, asking me if I wanted to start the installation of Windows 10.
The whole installation process took just over an hour, I performed the upgrade while I was watching the MS Gamescom conference. Immediately noticed when it had finished that the display drivers weren't installed, running windows update found the drivers but failed on them, though after a restart the display drivers had actually been installed.
I went and decided to reset my system, got a lot of junk on it and that took about half hour, all in all a fairly painless process.