Mountain Lion for me has reduced most of the serious issues. I was having problem mostly with system freezes when using Safari or Firefox - and sometimes neither. It didn't happen all the time, but enough that it was a concern. In fact, I had been getting more of them in the weeks prior to the ML release. Now, I have had no system freezes and an only ocasional beach ball from which the app recovers. Safari 6 reported a Quicktime plug-in crash, but the app kept working.
Over all the computer is faster and I really like the new ML features. I hope ML helps all of you having the really serious issues. Good luck!
I had very infrequent crashes before upgrading my MBP6,2 to Mountain Lion. Apple support over the phone said they are fairly sure it is the video card. I am going in tomorrow to the GB. I already had to take my MBP in to have the motherboard replaced once before for the same reason but that was within 6 months of my purchase.
I am not happy because even though I have AppleCare for another 4 months and this will be repaired by Apple for free... I use my MBP everyday and need it for work. They say it will probably take 2 to 14 days to repair/replace the moutherboard because the VC is built into the motherboard. Plus I have a feeeling this issue will rear its ugly head again once my AppleCare is over.
I expected more from Apple. Not sure if I will be getting another MBP when this one finally dies from its 3rd video card failure.