I have VMware Workstation but decided to try VirtualBox under Bea. I just wanted to let others know what I found so they can better decide whether they want to install it or not. The other free alternative is VMware Server which I'll quickly cover at the end of this post.
First of all, I was impressed by the speed and stability of the virtualization VirtualBox has achieved. VirtualBox appeared to run Windows XP Professional just as fast as VMware Workstation or Player. I did not experience any stability problems. This was quite impressive for an initial release.
However, VirtualBox is new and does suffer from one major drawback, poor device virtualization. It can't share most devices with other virtual machines (i.e. CD/DVD drives, audio, etc.) and CD/DVD drives are seen as VirtualBox devices and won't play encrypted DVD movies, allow CD/DVD burning etc. The only devices it seamlessly shares are the mouse, video, and network devices. NAT networking is easy to setup, while setting up bridged networking is a bit difficult but does work. The video card device is emulated very well, and although its also a Virtual Box device it seems to run as fast as VMware with experimental DirectX enabled (although it doesn't support DirectX).
However, Linux Mint is awesome, and provides plenty of CD/DVD burning and viewing programs, so unless you have a specific need to run a hardware dependent program most of these virtualization drawbacks shouldn't be an issue. Unfortunately though, the problem for many will be attempting to run more than one VM simultaneously. If you want to do this you have to manually connect/disconnect most devices as you use each VM.
This is really a pain if, for instance, you want to hear the audio from all running VMs. You have to physically connect and disconnect the audio device each time you switch machines. On the other hand, if you don't need to share devices simultaneously these drawbacks are not an issue.
So in summary VirtualBox is great if you only want to run one machine at a time, and don't want to run hardware dependent programs. It's not so great right now for running multiple VMs, although I know the developers are frantically working on it.
As for VMware Server, it provides excellent virtualization but the GUI runs very slow because all GUI data is sent over the network. However, you can create VMs with VMware server and then run the VMs on VMware Player and the GUI is capable of full speed operation, you can even play DVDs on the VM. The caveat is that you can't install VMware Server and Player on the same machine at the same time so if you want full speed virtualization you have to install VMware Server and create your machines, then uninstall VMware Server and install Player to run them. In addition to this, if you ever want to change the VM configuration (change memory size, add/remove devices, etc.) you have to reinstall VMware Server again to do it. All of this uninstalling and reinstalling quickly becomes a pain if you need to create or change VMs frequently. Remember though, if you don't care about the GUI speed you can just use VMware Server without Player.
Lastly, VMware Workstation, which I currently use, overcomes all of these problems but is very expensive. I bought it last year before the plethora of free products came out, and would probably choose the VMware Server/Player solution today, and await further development on VirtualBox devices.
OK, that's my two cents on VirtualBox. It takes a lot of time and effort trying all these virtualization products so I hope it will help some of you to better make a more informed initial decision, and save you some time.
Long live Linux Mint!