So I just upgraded my MBA 2011 11' i5 from Lion to Mountain Lion. I didn't do a fresh install and just upgraded on top of Lion. The first major issue I have is when scrolling in safari, whether zoomed in or not. It just seems laggy. Another issue is that all software updates are now from the App Store and the interface is a little different and it seems i cannot chose what I want to upgrade and what I don't. Another issue is that I can no longer see the time left on the battery but I can still see %. So after about two days I see the upgrade as completely useless. I don't even know how it is different or any better than Lion.