I used Parallels for a while but I was never able to leave it on without it eating up my CPU for no reason. I gave VMWare a shot just for fun and what I found was a much more stable experience...so much so that I can usually leave it on all the time without noticing a decrease in performance on the mac side:
I have just upgraded to VMWare Fusion and truly enjoy some of the new cool features:
Unite: Seamlessly run Windows applications alongside Mac applications with the Unity features in VMware Fusion. Find and launch Windows applications quickly with the VMware Fusion launcher. Switch between Windows and Mac applications quickly with Exposé. Minimize Windows applications to the Mac OS X Dock.
Play: Play Windows games on your Mac. Create powerful multi-core virtual machines and run 32- and 64-bit operating systems with ease. Use your iSight camera in Windows and gain access to Windows-only USB 2.0 devices.
Bootcamp: VMware Fusion makes it easy to install Windows as a virtual machine on your Intel-based Mac, and makes a perfect complement to Apple Boot Camp. Use your existing Boot Camp partition as a virtual machine, or use the built-in Windows Easy Install to install a fresh copy of Windows on a new virtual machine.
Snapshots: Use Snapshots to save the state of your virtual machine, and revert back to that state if your PC crashes or becomes corrupted.
VMWare Fusion 2.0 (currently in beta, but a free upgrade for Fusion 1.x users) includes even more:
* True Multiple Display Support
* support for DirectX 9.0 Shader Model 2
* Import Parallels and Virtual PC for Mac Virtual Machines, or Even Your Boot Camp Partition
* seamlessly print from your VMWare Windows partition to your OS X printers
* Faster USB Storage performance
* Improved performance when using NAT networking
