Until today, I frequently used vmware server. Now I switched to Virtualbox! Yeah!
My first attempt was a virtual Red Hat Enterprise Server. Virtualbox can use the .vmdk files from vmware, but that didn't work with this 20G virtual disk (split in 2GB parts). Putting the files together with
vmware-vdiskmanager -r RHEL52.vmdk -t 0 flat.vmdksolved that problem. RHEL53 is running excellent now!
Second test was installing a Fresh Ubuntu 9.04. This was really easy! I like this virtualbox wizard, it asks all the right questions.