Virtualization

So, I'm interested in hearing from anyone who currently supports an environment with a large number of virtualized applications. What are your gripes/likes? Are you moving to another product? Forget what they mfg says, how easy are the upgrades? If you have the vendor's suite, how does that help you? Were you able to justify the ROI/TCO to management?

I suppose I'm pretty interested in hearing about both the Microsoft and VMWare virtualization products the most, because from everything I've seen so far they look like the best, especially if you are a Microsoft / VMWare shop already.

So fire away! Thanks for your time all.