I use both Parallels and Fusion, but for slightly differing purposes. I find parallels better for virtualising a machine to use for things like your normal windows apps (Like office and the like) so I tend to use that as my 'Windows' desktop.
VMWare is far better though at running multiple OS at the same time, such as Windows Servers etc. I tend to do a lot of testing where I may have say 3 x Windows 2003 boxes running with SQL, Exchange and say OCS. That works pretty well with VMWare but really struggles with Parallels.
The integration with parallels (Coherence etc.) seems to work better than the VMWare equivalent.