I've used VMWare. I've used Virtual PC at work. I'm using VirtualBox at work and at home.
VMWare was cool. I used it at college and at one employer. At the employer we used ESX server, and all of our development machines were virtual. Cool idea, but things were very slow at times.
Virtual PC is good and has awesome features, like differencing disks and undo disks. Differencing disks let you make a base install, then create new machines from that in just seconds. This comes in handy when you need to spin up a new VM quickly. Undo disks let you install software to test it out, then when you close the machine, you can choose to commit your changes or go back to what you had before you started messing up your machine. :)
VirtualBox is great for day to day use. I use it at work and at home. At work, I keep an Ubuntu linux VM running at only 256MB of RAM. It's fast and when I close it and save the machine state, 256MB goes to disk in a few seconds. Likewise, when I resume the machine, 256MB loads very quickly--much faster than a fresh boot. Installing Ubuntu on VirtualBox was easy. Installing it on VirtualPC took jumping through some hoops...not fun. At home, I use Ubuntu, and keep an XP VM on VirtualBox to run Quicken basically. I also use that XP box for connecting to work because the Terminal Services client in Ubuntu is a little shaky when controlling machines within machines. For instance, when I remote a work server, then inside that remote my work desktop, the mouse is crazy slow. In any case. VirtualBox is awesome. I'm really happy with it, and when given the chance, will choose it over Virtual PC or VMWare.