I'm running the beta at joshuapettit.com and
www.mousefunfacts.com - no major problems, but they aren't terribly active and I've fixed all the known bugs.
My advice to clients asking me about 6.5 is to at least wait for b2. Unless they want to be paying me to fix every little thing - beta 1 has bugs.
Remember with a beta that bugs are expected, if you update it to your live site and find a bug, as far as rick's time, fixing beta bugs comes after support for the current version. So be prepared for potential problems.
If you're comfortable fixing bugs yourself, and/or applying fixes to your own version, then do it.
Otherwise, my advice would be to wait just a bit long, let some more bugs iron out, and wait for a beta2 release before doing a live site. Then understand and know all the risks of running a beta in a production environment.
