Parallels has enjoyed an early lead in the Mac VM market, but it is headed for serious competition from VMWare. I like Parallels, they are local boys and I wish them well, but VMWare is coming at them hard with Fusion. Its first release (it is in beta now) will include features such as Unity (VMWare's version of coherency mode), run from Boot Camp partition, DirectX 3D support, snapshot / backup, virtual SMB and the ability to run 64bit client OSs.
Both VMWare and Parallels have a feature that seems to have slipped below the radar but which I feel is the "killer app" for Mac switchers. On Fusion it is called the VMWare converter. On Parallels it is called Transporter. These utilities allow you to convert existing physical machines into virtual machines. This allows a new Apple convert to migrate his entire existing Windows machine (OS, software, data) to a virtual image and run it on his new Mac. Talk about reducing your risk. If I were Apple I'd be seriously hyping this capability.