VMware Fusion beta 3 with the debugging turned off is quite fast. I'm running Vista Ultimate under it (although the fancy stuff doesn't work, not that I like that stuff anyway). I have XP in boot camp and find that using VMware is fine for me, unless I want to play games. I used to dual boot Windows and Linux, and now dual booting OS X and Windows is just as annoying, especially with the clock thing making the desktop in Finder malfunction.
BTW, used to use Parallels, and used the Cohesion feature, it was good for a little while, but got sick of it. Parallels is good, but I'm a VMware veteran and prefer it.