I have used VMWare Fusion for a couple of years now and have no complaints whatsoever. It's stable and not noticeably slower than using Boot Camp. USB pass-through and networking just work. You can even launch a Windows program directly from Finder. My one gripe--slow access times for the host-client file sharing interface--was fixed with 2.0.
I have never used any other VMWare product, or Virtual Box, but I thought I would add my $.02 since my experience seems to differ from most others here.
