I was exactly in the same boat. I had ESXi 4.1 running for over a year with Unraid, ClearOS, and WHS2011 as primary VMs. Then it died on me two wks ago. Spent a week trying to upgrade to ESXi 5.1/5.5, gave up, decided to give Unraid/Xen a try.
I have so far loaded WHS2011 to it could not get ClearOS to load. I am currently looking for an easy replacement.