Upgraded my 2009 iMac from Lion (Lion install was done as a clean install shortly after release). No issues, runs nice and quick only been rebooted once since upgrade after I changed the location of my iTunes library.
No lockups/freezes or otherwise.
Macbook Pro late 2011 (SSD - HDD in Optibay) Clean install with Mountain lion. No lockups freezes or otherwise. battery life clocks in at 6-6.5 hours per day with 60% screen brightness.