I bought Mountain Lion via the App Store two days ago. At first, I thought it was the best $26 I ever spent. I loved Full Screen mode, I loved that I can finally get the latest updates on a number of apps.
Then, it hit me. My late 2008 MacBook Pro is crawling. Ok, maybe that was because it was still indexing, so I waited. In about a couple of hours, indexing was done and the MBP was still slow.
Opening Disk Utility was slow, opening tabs was a crawl, switching between apps was a nightmare. Oh my god, did I just killed my Mac? I started to verify my disk, repair my disk permissions, resetting SMC, resetting PRAM. This forum discussion helped too. Luckily, my efforts seemed to pay off. It seems to improve on the sluggishness but my Mac still suffers from the occasional slowdown for no discernible reason.
I shall have to trawl through the forums and see what else I can do.