VMWare Fusion is the bee's knees. I've been using it for the past couple of weeks and not only am I impressed with how well it works compared to other virtualization products on Mac OS X (it feels much more robust compared to Parallels), but also how well it works compared to other virtualization products on other operating systems (even VMWare Workstation on Windows).
VMWare Fusion's Windows Easy Install makes installing Windows painless; insert your Windows CD and VMWare does the rest, creating an optimized virtual machine and installing Windows (along with VMWare drivers) for you. It took about 30 minutes for Windows Easy Install to install Windows XP on a virtual machine on my iMac. Not bad!
VMWare Fusion's Unity mode is neat, too. Unity displays windows from the guest operating system on the host operating system (it's sort of like Parallel's Coherence, but better). There's a great demonstration of Unity on YouTube, if you're interested in seeing it in action.
If you're thinking of running Parallels, do yourself a favor and give VMWare Fusion a try. You won't regret it.