I installed Ubuntu Linux 9.0.4 in the previous version of Virtual Box.
(I also tried installing Windows XP SP2, but it would not accept my code, so I gave up on Windows, which I don't use anyway; just wanted to see if it works).
Ubuntu starts up alright, but because of VB's weird handling of the mouse (I have a USB mouse), there is no way of being able to enlarge the screen. I addition Ubuntu usually freezes half-way during a software update.
By comparison, VMWare Fusion Version 2.0.5 runs Ubuntu 9.0.4 flawlessly, allows a full-screen mode and interacts perfectly with the OSX 10.5.7 Spaces, so the transition to the Mac is painless.
It also seems to make better use of the graphics processor.
My iMac is the 2.93 GHz 24 inch model with the NVIDIA GeForce GT 120 graphics processor.
Windows or Solaris users may have a better experience with VB, and the fact that it is free and open-source makes it very attractive. I hope Sun will continue supporting it after it is swallowed by Oracle.