We have virtualized, I believe 55 servers, or 56. I can't remember. By the end of next year we will have nearly 100 virtualized. As others have said, backups are a breeze, failover is simple. We have no downtime on our systems that are virtualized because they failover to another running server with no issue.
The biggest selling point for us has been the actual dollars saved by virtualizing our environment. We're nowhere near fully virtualized, but we're getting there. We've used Hyper-V to create our own cloud in our data center. The actual monetary value we save each year makes this investment more than worth it.