Here's a list of what's supposedly compatible: http://roaringapps.com/apps:table I installed ML over my 10.7.4 CCC backup on an ext hd connected to my 2010 i5 MBP. It's still very early, but, so far...nothing jumps out as a problem. Such is the beautify of spare drives and partitions. Having said that, there doesn't seem to be any real need to upgrade on one's main system at this time. Hopefully others will post there impression here.