_place_holder;I upgraded my VMware ESXi 3.5 host this afternoon to VMware vSphere ESXi 4.  _place_holder;The process went smoothly.  _place_holder;I am sure that if I had take the time to read release note or some form of technical documentation, thing may have not been as unnerving.  _place_holder;For instance, the VMFS datastores were not added in when the upgraded host came up.  _place_holder;Not a big deal, and only caused a few moments of angst.  _place_holder;Everything was fine, and a simple add storage operation brought all of my VMs back to life.
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All in all, a very good experience.