Up until this week, I've always used either Windows XP or Vista as the primary operating system for my desktop computer. My laptop has OS X installed, which is enough for web browsing, email and access to our internal CMS. Someone once mentioned that Windows XP running under virtualisation on Leopard worked effectively. Indeed, using the Unity feature of VMware Fusion, you can run your Windows applications on the Mac desktop, alongside your Mac software.
Surprisingly, this is really effective. The Unity feature really does enable you to open, say, a Windows Explorer window right on top of your Mac desktop. Ok, resizing the window takes more redrawing than it would on a native Windows OS, but it's fast enough to use on a daily basis, enabling you direct access to both operating systems on one machine.
Only a few years ago virtualisation was too slow to be used for work purposes. Basic testing perhaps, but no more. These days we can run our Windows apps - along with their Windows UI - at a realistic speed on another operating system. I can't explain how interesting, useful and surprising this feature can become, until you experience it for yourself.
VMware Fusion 1.1.1 link.