Though very happy with Parallels
Get the best of both the Mac and PC worlds with VMware Fusion. With an intuitive Mac-native interface and a wide array of powerful features, VMware Fusion provides the most seamless way to run Windows applications on your Mac.
I am trying out the VMware Fusion 2 Beta on my iMac, it installed just fine as did Windows Vista.
I don’t do a whole lot in Windows, but for some stuff you need Windows such as GO!View for the PSP.
These OEM versions will work with VMware Fusion (and Boot Camp or Parallels). Though as OEM versions you won’t get any support from Microsoft, for that you will need to purchase the full retail versions.
Note that Windows Vista Home can not be used with Parallels or VMFusion due it licensing restrictions.