Ranked in these QuestionsQuestion Ranking
Pro Free and mostly open source
VirtualBox is available for free with source code available here. A "VirtualBox Extension Pack" that includes virtual USB 2.0, VirtualBox RDP & PXE boot support is available only under a commercial license.
Con Confusing interface and lacking usability features
VirtualBox might be a bit confusing for beginners - it requires an understanding of a lot more technical details than other VMs.
Con Inferior performance
Apart from second-class performance in virtualization, there are some visual glitches with Windows 8.1 on Mavericks.