I trade from a MacBook Pro, most of the time hooked up to an external 24" display, running Windows-only applications (like eSignal) under VMware Fusion with Windows XP.
Originally this was planned as an backup only for my PC (and because a wanted to try out a Mac), now the PC is the backup. It's just a matter of time when I will go 100% Mac. One thing is for sure, I will never return to a crappy Windows PC - almost get aggressive by now if I just look at one.
XP under Fusion actually runs better than on an normal PC, because you can simply save and roll back installations when testing new software, or copy virtual machines to test different configurations and so on. This results in a much stabler system.
Regarding the so called "mac premium": Think in total cost of ownership, not initial cost.