I upgraded within a few days of the first/original release of Mountain Lion and haven't experienced 1 problem since my upgrade. I'm using a 13" MBP that is a couple of years old. That being said, everyone always seems to have a different experience.
My battery life did "seem" to be less than with Lion. But, I could be imaginging this. I often run my laptop directly via household current vs.. cycling the battery like many people do. I'm really not that portable.
As of right now, after this most recent update, my battery life indicator shows that I have 6 hours left at 97% charge. That's with Chrome and Safari running.... Chrome having 10 tabs open and Safari having 2 tabs open. Brightness for the screen and keyboard is at 50% each. Battery cycles indicate 494 with battery health at 86% (4972mAh). Using after-market SSD.