VMware Fusion is the best choice for running Mac OS X Server in a virtual machine -- a very handy thing to do for testing a server before rolling it out. Installing Mac OS X Server versions 10.5 and 10.6 was trouble free; the server operated just fine running services on the network, and it didn't require futzing around with virtualization settings.
In addition, Mac OS X Server ran faster in VMware Fusion than in Parallels Desktop. Parallels was downright finicky with Mac OS X Server. I only got it to work after installation failed several times. Parallels was also unable to import a Snow Leopard Server virtual machine from VMware Fusion, although it does support moving Windows and Linux virtual machines from Fusion.
Where VirtualBox falls short VirtualBox provides good basic virtualization, without the ease-of-use and polished user interface of VMware and Parallels. But you can see the differences between VirtualBox and the others as soon as you install a guest operating system. Parallels Desktop and VMware Fusion automate the installation of Windows, filling in user information and advancing the process. With VirtualBox, it's up to you to click through the screens and select the information.
Importing existing virtual machines from other sources isn't straightforward. For instance, if you went directly to the Import command, you'd be headed in the wrong direction. And while VMware and Parallels support the importing of each other's virtual machine formats, VirtualBox doesn't support importing the current virtual machine formats of either Parallels Desktop 5 or VMware Fusion 3. (VirtualBox supports Desktop 2 and Fusion 2.) I was able to import an Ubuntu Linux virtual machine downloaded from the VMware Website, but only after experiencing problems with a perpetual spinning beach ball that left force-quit the only option.
[ VirtualBox fares much better against VMware Workstation and Parallels Desktop on Windows. See "InfoWorld review: Desktop virtualization for Windows and Linux heats up." ]
Once you're running a virtual machine, you won't find the interface niceties of Fusion and Parallels Desktop. For instance, getting out of full-screen mode is done strictly by key command. VirtualBox's answer to Coherence and Unity is called Seamless, but it's not as integrated into Mac OS X as VMware Fusion and Parallels Desktop. Seamless simply hides the guest operating system's desktop, while keeping the Windows (or Linux) task bar at bottom, just above the Dock. Sometimes it appeared behind the Dock, which isn't all that useful. Windows applications don't get individual icons in the Dock or in the Application Switcher, and they don't integrate with Expose.