Fusion was very buggy for me on my unibody Macbook. High CPU, very slow unity, occasional OS lock-ups. I tried Parallels 4.0 the latest version like 4 months ago and is way better than the current version of Fusion.
For starters you get the Windows tray icons on the menu bar. The icon on the Dock changes to a start menu when in Coherence (unity mode for fusion), so you can access it even when native Mac applications are selected. With Fusion you have to choose an active Windows app to get the menu option, so you open another Windows app from the start menu while in unity. Parallels uses less CPU and memory and the Coherence mode is faster. Parallels supports simple cloning and cloning to a template much like VMware Workstation and ESX. Cloning is great if you need to make multipe VM's of the same OS. With fusion you are left with coping the vmdk file, and creating a new VM. The sharing options between VM and OS X, files, folders etc is just way better in Parallels, I get a disk icon on the OS X desktop for a running VM, that I can access. Parallels supports Snow Leopard and a 64bit Kernel.
We have been beta testing Fusion 3.0. It is better than 2.0, way better. The windows start menu on the OS X menu bar is huge for me. The performance of Unity is better as well. The sharing options are lacking still. The cloning options have not changed.
I will probably switch because we are such fans of VMware products over all. If I were not a user of other VMware products I am not sure I would. Aero and gaming do nothing for me. I only run Windows in a VM because I need VMware tools or MS AD tools.
What VMware really needs to do is make some native Virtual Center tools for OS X, so I dont have to startup a Windows VM at all.