What we used it for was for "sand boxes." That is each vm was the owner's playland and they could configure and crash their system without bringing down the entire web service.
Beyond that I've found little use for VM.
Hey Folks - We need your help. We were hoping, we could gain from your experience with implementing server virtualization. When should we be using Red Hat Virtualization, Citrix Virtualization, Xen Virtualization, Microsoft Virtualization, Oracle Virtualization and VMWare. I will be very interested in learning from your first hand experience. I will also like to know, under what situations is one virtualization preferred over the other. - Thanks for your help