VirtualBox is, nominally, open source software, though you have to specifically download the ASE version to actually use the open source client. The proprietary one is not much different, so it's really a matter of your personal philosophy.

Setting up virtual machines with VirtualBox is a little less intuitive than in VMware or Parallels. It wasn't entirely clear at what point the process called for connecting to an existing ISO image to use for creating the new VM. It worked all right, but less-advanced users might have trouble parsing out the steps at first try.

One feature I missed: there's no interface integration tool in VirtualBox like Unity and Coherence. Everything runs inside one virtual window.

On the other hand, a feature I really liked was the capability to create VMs that could be used in other virtual clients. I walked through creating a Parallels virtual machine in VirtualBox, moved it to the Parallels test machine, and the VM opened without a hitch in Parallels.

Given its cost, and the fact that it is a true cross-platform client, overall I would recommend VirtualBox to anyone who needs virtual machines that set up and get moving quickly.