Here is a good list of the pro and cons: viewtopic.php?f=2&t=535
I personally tried ESXi, but switched to Bare-Metal. I think that a big advantage of NASes is redundancy, and the problem about ESXi is that S.M.A.R.T data is not available to DSM. Thus, the hypervisor has to handle disk failure, and I have no clue what happens when a disk fails.