Performance / ESX / VMware
ESXi has rather narrow hardware requirements, and the new management and consoles of both VMware 2.0 and ESXi are weak at best. Having an actual full blown OS underneath your HV adds a bunch of flexability to your mix. VMware currently has the market because it's tools were much better, and it's products were more stable. The latest console fiasco is stunning IMNSHO.
VirtualBox (Sun) allows one to set up headless VM's that console connect from any RDP client. This is a Huge win IMHO. Further I've found I can run VirtualBox and a couple of VM's on very modest hardware, like a 1.6 Atom with 2GB of RAM running Slackware and XP on top of Ubuntu9.04, something I was flat unable to do with VMware. Someone enlighten please, Will Microsoft's product run anything but Microsoft software? I would truly be suprised if it did.
Long term if VMware wants to hold market share, they need to lose the sorry console client, and do something about the performance. VirtualBox is not there yet, but a little polish and it's going to eat VMware for lunch. I've re-scripted the startup/shutdown for VirtualBox, and have 7 VM's running on some heavier hardware. I think VMware is getting the IBM throw more hardware at it mindset., where you add more blades.