I went from 10.6.8 on my early 2008 MB to 10.8.1 (and now 10.8.2) on this late 2011 MBP. There are a lot of UI changes that take some time to get used to. For the first week, I kept triggering unexpected actions with gestures I didn't realize i was using. I haven't had any issues with software compatibility. The only major crash I had was when I was attempting to do a full Time Machine backup of the system's HDD and a massive rsync between two external drives. The machine recovered fine from it. In general, I quite like Mountain Lion. I never used Lion, but by all accounts ML is a significant improvement over it.