Never underestimate the power of simplicity...
I've been running my Windows environment for work purposes on a Virtual Machine.
I started off on Virtual Box and decided after a little while that I wanted to see what more sophisticated virtualisation would be like.
My first attempt was Parallels 5. Now Parallels offers fantastic Integration, for example it wraps all the running Windows processes and presents them as processes running in your environment, so you can switch to VM applications as if they were running inside your host OS.
The only problem was that Parallels is dead slow. It managed to grind my quad core i5 Macbook Pro to a halt, not to mention all the daemons it runs messes around with the startup and shutdown of the Macbook itself.
My next attempt was VMWare Fusion. Now VMWare is the Mercedes Benz of virtualisation products, and although it wasn't as fancy as Parallels, it worked very well, easily reading and importing my existing virtual disks, and running the VM with little fuss and overhead with all the integration I really needed, without dragging down the performance of my Macbook.
Unfortunately, on day three of my VMWare experiment I tried to restart a paused VM and it crashed and would not let me restart the OS inside the VM, I tried to reimport the original VirtualBox VM but it would not let me.
So now I'm back on VirtualBox. It's fast, simple and so far the only Virtualisation product I've been able to use with any success.
Bottom line: Simple is best.