This is getting off topic, but I think ESXi and friends are awesome so can't help replying...
I run Citrix XenServer at home, and my father and neighbour run ESXi. We all use these to run a single home server VM 24/7, and that's about it (so only one live VM!).
It's dead easy and quick to set up, you can use cheap parts with local storage (with the right motherboard for ESXi), and you can stick the machine down in the basement or in a closet. From then on you don't have to mess with the physical machine.
Once every year or two we'll be upgrading (software-wise) or reinstalling the home server, and it means we can have the new and old virtual servers running side-by-side and just transfer stuff across. All without spending extra on hardware.
Dad likes playing with Linux, and my friend and I wanted to try out Windows 7, so it can also be used for things like that. I know you can do that with Server, Player, Fusion, Parallels, Virtual Box etc, but I like that I don't have to have extra network interfaces/drivers on my computer that might mess up other software (games can be picky about it), and that I'm not filling up my personal harddrive with that stuff. Using ESXi or XenServer is less hassle for us. It also means the test machine can be left running when I turn my computer off, for example if I wanted it left on downloading stuff (which is nice because my PC is noisy, so having it on all the time is a pain).
Anyway, we all really like the ease and flexbility doing this brings. We don't need it, but it's nice. :-)