I installed a CentOS5 guest on OpenSolaris desktop using VirtualBox and played with it a bit. The installation process is smooth. The hard disk and partitions are hidden well by VirtualBox from anaconda, thus making the entire installation process very simple. Even the network configuration, display card configuration, etc. are very simple and straight forward.
I had a crash of the OS and the virtualbox while playing with several tools on CentOS, but could not reproduce it again. Well, it sounds okay for a beta-1 build. Tools like browsers, network monitoring tools etc. worked very fine on the guest OS. I may look around for Windows and other OS DVDs. That would overload the host OS and helps me see the performance impact.
I liked the way mouse capture and keyboard capture are toggled. Overall, it is impressive.