Upgraded to MC6 on a mac with new os 10.7.2 from 10.6.8 & 5.5.x. I have Kona Lhi card and did all the recommended driver upgrades in the recommended order. Now my last project edited on 5.5 is corrupt. It's a project with several sequences and lots of raw clips that are AMA linked. I get an error message "serious error has occurred. Save and restart your system" whenever I try to playback a sequence. I can playback clips in the source monitor and see them on my HD monitor, so that all works. I think it's probably some project settings. I'm wondering if there is an easy way to trash the project settings without screwing up the bins and sequences?
I've "gone back and forth" with dozens of projects between 6 and 5.5. Never a single problem. Sorry to hear something didn't work for you, but my experience would say that it's not related to software versioning of any kind.