I have been using 8, and 8,1 since they came out; mostly in Parallels, and in Bootcamp from time to time for gaming. I have not run into any major issues; one time there was a glitch with sound. The biggest issue for me is lack of support for numerical keys on the Apple USB keyboard, which I need for work.
(Edit: the numercial keys work within Parrelels, but not using native Bootcamp.)
I second this. I have three VMs, Xp, 7, and 8.1. 8.1 is by far the fastest one, even with SQL server 2014 operating with the management studio in use too. I use dual screens in full screen mode.
Y'know... Windows 8.1 is damned fast, and pretty low impact on resources. If they would dump that tile screen, they'd unshackle themselves from a lot of ill will to reveal a pretty banging improvement over 7.