ESXi? Woo. Hoo.
We took a look at ESXi recently. Big fail. The only hardware it runs on is too expensive to be worth it. And yet ESX, the paid-for version, runs on much cheaper and older hardware that can be bought on eBay for a small fraction of the price. VMWare's generosity has underwhelmed me.
We're evaluating VirtualIron now - much cheaper than ESX, and runnable on any CPU with virtual support, so more-or-less any desktop with an Intel Core processor will do.
With VirtualIron on servers, and Sun's VirtualBox on desktops, VMWare's starting to look a bit expensive at all levels.