I just upgraded a VMware cluster from version 3.0.2 to 3.5 (2 nodes connected to a SAN). It was very easy, well documented and a very pleasant experience. Of course, in reality the ESX service console is really just a customized Red Hat Linux installation, so the upgrade for that section is really just an upgrade of all the RPMs. The VirtualCenter Management Server (a Windows application) was also well designed and tested multiple issues with the database before attempting to upgrade it. The tests actually found several issues with the database settings from the original 3.0.2 install. The ESX server upgrade was done by booting off of the upgrade CD, however, next time I would like to try the esxupdate utility which appears to be something similar to the yum or up2date utility.