I've been using it exclusively for a few months now. (except for a few projects or tasks where Win7 or WinXP is needed).
Enough time on the 8 that it actually seems 'normal' and I have to watch myself when I start to answer some questions here in the 7 forum, for example.
For me, it's been a smooth ride. It's been a much easier transition than going from NT 4.0 to WinXP for example.
I have Metro Start mapped to an extra mouse button and I can flip back and forth between Metro and Desktop a bit faster than I could do Win7 Start Menu stuff.
I'm not going to recommend anyone already on 7 to upgrade. But in few months they'll be selling systems with just Win8.