I tried the beta, upgrading over my existing version. After the upgrade, I couldn't get my Vista (RC1) to load. I'd get the status bar, then it would freeze and my memory/CPU utilization went through the roof. I could still control Parallels, and other items, but Vista wouldn't load.
I downgraded from the beta to my original version (1970 I think) and Vista loads normally. I did have a few scares, where the Vista image 'disappeared' and I had to use Spotlight to find it. But all if okay for now.
Perhaps when my 'real' copy of Vista appears, I can try again with a clean install of both Vista and the beta of Parallels. For now, I'll stay where I am