I use a mix of WinXPPro and Win2K3 in a variety of environments at work and at home. I use Win2K3 in both workstation and server roles.
At work I use Win2K3 as dual monitor workstation for example. As a software architect having the featureset of our datacentres available on my desktop is an absolute must for developing proof-of-concept approaches, and of course familiarity with the available services. With the process priority set to foreground interactive applications Win2K3 is as responsive as XP.
Of course the hardware for Win2K3 is a little more needy with more services enabled, but RAM is cheap and even my dual 550 Xeon server at home runs it fairly well.
There are very few reasons beyond licence cost not to run it, most applications run unmodified, most that don't only need XP compatibility set on their installers and work just fine when run. I generally had more problems getting a couple of device drivers for my older server (circa 1999 - couldn't get an updated ACPI driver that worked) than newer hardware.
There is no upgrade path from Pro - it'd be a side-by-side install. Try it and see for yourself...