Parallels suffers from upgrade-itus
My experiences with Parallels on the Mac have been less than satisfactory.
Upgrading an Ubuntu VM to 9.04 caused it to become unuseable; apparently the way that Parallels virtualises the video hardware is rather shoddy. New versions of xorg can cause it serious problems. This has been an ongoing problem for Parallels.
I've heard that Parallels *still* doesn't have support for Ubuntu 9.04.
We changed to Virtualbox which is already cross-platform; OSX, Linux, Windows. It works a treat and its free, even for corporate users (provided that its not mass-deployed).
Oh and it seems to be somewhat tricky to get off of Parallels spam^H^H^H^Hmailing list...