We deployed a large multi-state 5 city project using ONSSI RC-E software using Virtual Machines. The client provided Flexpod and VMware machines. This was very successful with a few bumps in the road.
Advantages are numerious.
VM vs Pizza Boxes VM is dynamic, if you need more CPU you allocate it, if you need more storage you allocate it. Rack mount machines only have the CPU's you bought or physically change the chipset.
With VM you can allocate any of the resources in any fashion as you need them. Slice it and dice it any way you need.
Hot standby is a big advantage and the cost is minimal as opposed to a hot standby physical machine.
One of the big problems was disc availability on boot up and disc latency. If the machine looses its discs you have paperweights. With the proper configurations this is not an issue.
ONSSI was very helpful throughout the process, they even studied the setup for reference.
One big thing is planning, you have to plan with the hardware providers, the software manufacturer, the client, PLAN, PLAN, PLAN!
"In preparing for battle I have always found that plans are useless, but planning is indispensable."
Dwight D. Eisenhower
Todd