before joining microsoft i was a technical trainer and part of that job was dealing with the constant "what if" questions that would come up in a microsoft course ("what if you created or deleted an object from two windows 2000 active directory domain controllers at exactly the same time?" - a personal favorite). sometimes this resulted in learning the answer and nothing more. more often it resulted in a change that i wasn't anticipating later in the course (that would mess something up or cause people to do more than just "follow the lab manual"). i was familiar with vmware from a previous job and inquired about pricing for technical trainers (in 2001) - vmware didn't have any kinds of discounts, so prior to microsoft acquiring connectix, i started using virtual pc. i never used the virtual pc on an apple computer (the first apple computer i owned was an intel based macbook that runs windows) but have since used virtual box, parallels and vmware fusion as well as virtual pc (bootcamp is dual boot so it doesn't really count). the fact that i can virtualize the operating system means running test operating systems or evaluating beta applications like internet explorer 9 is very easy (and can even be "undone"). lately my favorite client virtualization app is not from microsoft for one simple reason - 64 bit virtual os support.
at work i have made the transition to hyper-v (the server equivalent of client virtualization) which supports 64 bit virtual operating systems, but at home i don't want to run a server operating system on most of my systems (but still need 64 bit virtual support for testing/training/etc.) - so my product of choice has been parallels. the best part about it is i can run software like zune, internet explorer and office 2010 on my macbook, my laptops or my desktop computers!
Like you I've used them all at some point or another but Parallels is the best non-server virtualization product on the market and at the right price!! Especially now that almost everything is 64bit. I've got it loaded on my new 64bit 6 core box w/64bit Windows Ultimate and can set up anything (starting a POC enterprise design) I want whether 32 and/or 64 bit. A real shame Microsoft hasn't done anything like this.
Darrell - thanks for your post! Giving examples of non-Microsoft apps that I have/do use was one of my goals and when the applications work as well as Parallels, I don't mind showcasing them here. For most non-admin/testing I only virtualize 32 bit (and run 64 bit physically) but when testing software, it's nice to be able to virtualize the environment. You can boot from a VHD of 64 bit Windows and then it's running "bare metal" (only virtualizes the disk controller and partitioning concepts) but it's an extra step and a lot of the time I don't want to have to reboot.