I am still using Lion myself -- but I'm not aware of any issues and I haven't seen any posted to t his forum.
On the other hand, the OS is just a few weeks old.
There are several Mountain Lion posts on the PrPro CS 6 Forum: http://forums.adobe.com/community/premiere/premierepro_current?view=overview. I have no idea if any of those would apply with PrE, but it might be worth a little time to read. PrPro and PrE share some technology, but in general, they are totally separate programs.