There seems to be a lot of fanboy affinity around ESXi - and with the fact that its 'available' off the shelf, zero cost up front. however to make it do anything you still need to buy into vmware tools.
Huh, Tools are free? I do _a lot_ of nothing apparently without what I assume you meant, Virtual Center.
ESXi, like all vmware products, is highly polished and very reliable. I am a HUGE fan of Xen, spent a lot of time learning it and I love it. But don't discount ESXi since it's not open source.