I loved Mountain Lion and also found it to be very stable. I stayed on it as long as I could, but the new versions of several of the apps I use required Mavericks, and after holding out for a while I upgraded. Mavericks was when I started noticing issues... Graphics driver issues, trackpad gestures randomly not working for periods of time, WindowServer crashing when opening the notification center (wat?), so on and so forth. Mac OS is still my favorite, but I've gotten an irking feeling that in the interest of capturing the mainstream market and making things look pretty and full of features, Apple has skimped on stability and quality. Just my two cents