I have not tried VirtualBox, but I love that it is open-source. If it works as good as VMWare I will probably never use VMWare again.
I haven't tried VirtualBox either. Looking at it now, the one big reason I won't be using it is the lack of 64-bit OS support. In addition to that, other reviews and comments about it are indicating that it's not as mature or stable as VMWare. So far, VMWare is the best choice for me because it's not just limited to one OS (unlike andLinux), supports 64-bit OSes (unlike VirtualBox), and can interface with the necessary hardware without requiring a complex configuration process (unlike Qemu). I have a special soft spot in my heart for software that "just works".
VirtualBox probably wins out over VMWare Player in the area of additional features, since VMWare would like users to pay for the commercial edition to get those features.