I really like the Virtualbox virtual machine. However, there are two issues with it running on Ubuntu Gutsy. Firstly relates to file sharing and the second with USB.
Firstly, Virtualbox has a really cool sharing interface. However if you set up it with the default name, then you will get an protocol error when you try to mount it. (Ticket #928) The suggestion to get around it is to make sure you the share name is different to the folder name you want to share.
Regarding the USB issue, there is a cool howto on Virtualbox's website to set this up for Feisty. The howto explains you need to edit the file /etc/udev/rules.d/40-permissions.rules and give the user permission to access usb_device.
However, this alone will not allow you to have access to USB in your virtual machines, some files need to be created in proc. (Ticket #747) This is done by editing the file /etc/init.d/mountdevsubfs.sh and uncommenting the block "Magic to make /proc/bus/usb work". Once these two things are done, you will need to reboot and then Virtualbox should work fine.