I upgraded to ML with no problems. The whole process took about an hour, I think. But, it was fine. However, I noticed that in my disk display in iState menu showed a pretty considerable difference between used and unused disk space. This confuses me since I'm not sure which to believe. I read/thought ML was already compatible with iStat menus. Here's a screen shot showing the difference in unused/used space between my SSD from Disk Utility and iStat Menus.