You might be able to help me out here. I've used virtuals for medium server farms (1 app server, 2 app servers in failover, cluster of 2 DB servers, and a few more), and so far I haven't found anything I can't do with MS Virtual Server. But then I don't know if I'm even pushing the envelope to where the differences between MS and VMWare would show.
Any reason to recommend VMWare over Virtual Server except for VMWare running on other host OSs besides Windows?
This concerns both the daily maintennance stuff, like creating, cloning, managing and backing up virtural instances, as well as more sophisticated features like 'vmotion'.
What vmotion lets you do is take a live system, and migrate it from one phyiscal server to another while the system is live. Intellegently implemeted, this could virtually eliminate downtime for anything but actions which require a boot of the image, like OS patches. We're working on gaining confidence of this feature in our production environment, though the more paranoid among us haven't really bought off on it yet. However, once you do it, you can maximize hardware utilization by having servers move hither and yon based on load. For instance, if you've got a server that does serious batch work at night, it might not be allocated to a given piece of hardware during the day, but instead is migrated there at night to take advantage of the fact that some of the other servers are less active at night.
Our MS-Windows team looked at the MS offering and quickly rejected it as inadequate to the task of where we wanted to go with virtualization.