Graphic performance in Seamless mode can be a bit unresponsive, and it was a bit jerky when dragging windows around. And this is without Windows' hardware-hungry Aero feature, which VirtualBox doesn't support.
For Mac Pro users with eight processor cores, VirtualBox 3.1 enables users to assign up to 32 virtual processors to a single virtual machine, compared to 4 for VMware Fusion 3 and 2 for Parallels Desktop. Assigning multiple virtual CPUs makes a virtual machine run faster. But because you can't assign more virtual CPUs than the number of real processing cores, most Mac users can't take advantage of this feature. For all Macs except the Mac Pro and the Xserve, 2 virtual CPUs is the most you can assign to a single virtual machine in any of these virtualization products. With VirtualBox, when I tried to assign more than one virtual processor to a virtual machine on a dual-core MacBook Pro, a message stated that the setting is "non-optimal."
With Version 3.1, Sun added a unique feature called Teleportation. This is the ability to copy a virtual machine from one computer to another while the virtual machine is running, regardless of whether the host is a Mac, a Windows PC, or a Linux PC. It's an interesting technology that could be useful in enterprise situations, but the average Mac user is not likely to ever work with it.
VirtualBox's most compelling feature is its price: free. If you only need to run Windows XP or don't care about 3-D graphics, and you don't mind not having the Mac OS X integration features, VirtualBox 3.1 will work. But with list prices of $80, Parallels Desktop 5 and VMware Fusion 3 don't exactly break the bank.