In an article at Search Server Virtualization on VMware Server, a VMware customer had this to say:

"I would say VMware Server is a great way to get your feet wet with virtualization," Villalta said. "Where I see the server product kind of lagging is with products like Virtual Iron, where you can run higher-performance virtual machines. That's where I've seen the server product fall behind. Not on the utilization standpoint, but on the high-performance standpoint."

and

"You can run Virtual Iron and XenSource at the kernel level and they provide a higher level of performance," Villalta said. "That being said, VMware Server does give you flexibility. You can install it on any hardware and make that the virtualization host."

The full article is here.