I've used VMware Workstation for about a year at work and think it's one of the best pieces of software available for Windows (along with the mighty Visio). I'm looking into consolidating some servers using ESX, especially now the price of ESX 3 is so much more reasonalbe than version 2.
I've also done a bit with VMware Server, initially using the beta version and now the GA. This is the product that was formally known as GSX server. Unlike ESX, it requires a host OS which can be Windows or Linux. At work, I've used it to host a Solaris 10 installation which has been configured to run Jumpstart Server (for network installs of Solaris servers). At home, I've used it to play with ReactOS and Syllable. Today, I've downloaded and installed Ubuntu (to see what the fuss is about. Quick review: Looks easy to use, hate the default brown!) and OpenFiler (www.openfiler.org) is installing as I type this journal.
At the moment, OpenSuse 10.1 is downloading, which I'll test in a VM. If it works as well as the reviews and looks as good as the screenshots, I might be tempted to replace my Mandriva install. A story for another time.