VirtualBox has been my favorite desktop virtualization tool for a couple years now. I love it so much because it supports variety of host and client OSes, USB support, and shared folder with the host. The fact that VirtualBox application itself and virtual machine images are portable between my Windows environment and Linux environment is the greatest feature for me.
However, there have been some issues that annoyed me a lot.
First issue was when you run multiple guests, sometimes it get frozen and you need to hard-reset your computer. This happened quite often and it was very annoying.