CNET's Crave blog beat me to the punch and published a comprehensive article comparing Parallels Desktop and VMware Fusion, virtualization software for OS X, as well as Boot Camp and plain OS X. As you might expect from reading my VMware Fusion launch post, Fusion performed considerably better than Parallels Desktop - performing close to 4 times better than Parallels in a Quicktime video/iTunes audio encoding benchmark (which took advantage of Fusion's support for multiple cores).
If you were trying to decide which one to buy, I think your decision has just been made.