Stable? Yes! Supported, no.
I've only touched my toe in 0710 and promptly installed 0810 to have a "development" box, and I've gotten most things working. Some important features don't work yet, like "Manage Drives" for hard drives (works with my DVD-ROM drive), and I don't have any HA stuff at the moment. Basically, I've been able to play around with it very well. It's not mission critical in my house right now, so if you have a working 0710 running your house, DO NOT just upgrade on top of it. If anything, get a new hard drive and install onto that so if it doesn't work, just unplug it and boot up your known good 0710 drive.
I'll shortly have a tuner card (if it ever comes in the mail!) so I'll be able to report on Myth at that time.
And remember, it's not called "The Bleeding Edge" for nothin'!