At Silva Tech Solutions, we fully use Vmware for most of our virtualization projects. It has always been rock steady, feature rich and relatively easy to use through each version. Our only gripe has been the licensing restrictions that are very cost prohibitive for a majority of small businesses. By itself, the ESXi platform is fairly cost beneficial (free), but if you need to add any sort of reliancy to the network (failovers/replications/etc), you immediately need to start purchasing some very large license costs.
After having starting working with Xen Server (Here Feature List), there is very little more that a small to mid sized business would require that is not supplied by XenServer.
