I’ve been playing with virtual machines in OS X and my experiences have not been at all good.
Parallels 3 works well for running my boot camp XP partition but I have learned to upgrade with care and not to use the upgrade from within Parallels itself: download the upgrade from the Parallels site, upgrade, reinstall tools, works best. Parallels however was one of the suspects for trashing my boot camp partition after an disk change: the evil Spotlight was the other suspect. I had backups so I was able to backtrack and repeat the installation again but this time avoiding running Parallels until after Spotlight had stopped doing what is does best - insisting on indexing and using up way too much CPU when I don’t want it to run at all.
Parallels 4 was a release to far from all that I have read so I’m waiting a while before upgrading.
I have used VMWare to run BeOS Max - works reasonably well with a few issues: CivCTP upgrade will not run (blank screen), dodgy file manager in Max. Network works well. I have two instances running, one on the Macbook Pro and one on the Mac Pro.
VMWare is running instances of Debian 5 and Ubuntu 9 (both minimal installs) with tools installed. The tools allow me to resize the screen but the X fonts don’t scale well for browsers - I get a big print edition. The Network works well as does USB.
Virtualbox 3 runs instances of Debian 5 and Ubuntu 9 (again minimal installs) with tools installed. The screens resize well without breaking the font sizes. USB does not work reliably so USB drives may or may not be recognized, and if these are then these may not be write protected, even if lacking write protect: broken. The network is not reliable: I switch between WiFi networks or USB 3G broadband and cannot be sure of the virtual machine continuing to have network access using NAT. Use the Intel network emulation - the default is worse and will fail mid net install of Ubuntu or Debian
My conclusion is that these products are not really ready for mainstream easy use. Running the nasty windows os may provide better experiences but I suspect that the main issue is OS X support for VMWare and Virtual Box.
I may try virtual linux in Parallels next - just for fun.