Mountain Lion did not work for me. As an author, when I got the spinning beach ball of death for four seconds every time I cut and pasted even, say, just one character, it was time to go back to something stable. (The processor showed it was running at about 7%!) Switch it off and it worked fine...then after three days, it would slow down again. I've had Macs since '91 and I could not figure it out.
So, back I went and I'm happy with Snow Leopard...except that I've got 10,000 emails which Snow Leopard's Mail will now not recognize. They are on the drive and I can access them via Spotlight but nothing is in order anymore. It takes me ages to find what I am looking for. Please, is there a way to get them back into Mail? Some conversion program perhaps? Thanks.