What on earth does this have to do with the Amiga?
My opinion:
VMWare Workstation works well, it also seems faster than Connectix/MS Virtual PC. I've evaluated both on the same hardware and VMWare was a lot quicker in getting the OSes installed, etc. VMWare also provides OS extensions (and the actual VM) for Linux, which Virtual PC doesn't..
VMWare's virtual network is also a lot more customizable, but I've heard many reports of it's DHCP server starting to broadcast on the wrong interface, screwing up a few corporate lan segments until it was tracked down and disabled.
If you intend to virtualize Linux, I'd say go for VMWare.. OTOH if you intend to virtualize Windows (and the host is Windows), you ought to evaluate Virtual PC as well. Both give out a demo version if you're really interested.
I don't have that much ESX Server experience, but from what I've tried it, it seems quite nice as a virtualization platform.. It's basically Linux with the VMWare ESX software running on top.. You see the virtual machine's video with a remote console application and you can manipulate the actual VM Host and the VM's with a browser UI..
Now then, your turn: are you affiliated with VMWare or why did you paste the ad to Amiga.org?