Mmmm... the crashy goodness of VirtualBox but for the Enterprise.
I use VirtualBox for it's leading 3D support (3D acceleration in Linux makes it look and work a lot better) for Linux & Windows aero (which they finally sorted out). Free is a wonderful price too, but if someone suggested running something mission critical on VirtualBox I'd just laugh.
Only yesterday one of my VM's became inaccessible after I upgraded to the latest release and attempts to restore back to a previous snapshot failed. I had to clone it and terminate the original to get the VM working again.