I've worked with a number of virtualization products including VMWare and QEMU. While they are fine products and definitely get the job done, I've always had two complaints about them. First, they are tough to configure. In a lot of cases, getting a virtual machine up and running is time consuming and frustrating. The second complaint is the footprint it leaves on the system. VMWare in particular uses up a lot of system resources and can take forever to get loaded. Now, VMWare folks, don't get angry. I like your product and it is definitely industrial grade.
That said, I like VirtualBox from Sun a whole lot better. To begin with, it has a very small footprint of it's own. It loads quickly and takes up few resources. The interface is clean and easy to use. It has wizards that walk you through setting up virtual machines easily. There are presets for different types of common operating systems such as Linux and Windows variants. Setting up virtual disks is a snap and connecting to CD Image files (commonly known as ISO files) is equally easy.
Does this make VirtualBox the most robust, industrial grade virtualization system for use in data centers? I dunno. For the casual user who might want to do some cross OS development or a QA engineer testing on various platforms, this is so much easier than anything else I've tried. You don't need expensive training. You don't need the "Enterprise" addition to get anything meaningful done. And you don't need to read a manual the size of the New York City phone book or hire a consultant for the equivalent of the GNP of a small country just to do the basic stuff.
Some of the ubergeeks out there will probably assume I'm just stupid. They will argue that VMWare and its ilk is plenty easy. Argue all you want. I've used the others and this is, by far, the easiest to use and the quickest to deploy.
Besides being easy, it works very well. I have installed a variety of common operating systems, including various Windows and Linux systems, as well as more unusual ones like OpenSolaris and FreeDos. With the exception of OS/2 Warp (I don't have a floppy drive and the OS requires one to load) and Windows 98 (still don't know what that was about) everything installed flawlessly. I can't say I've had that same experience with other virtualization software.
Oh, and did I mention it was free. That's right. It Open Source from Sun. Sure, if I was deploying this in a data center I would pay for the service and support. At least I would have been able to fully evaluate what I was paying to get serviced and supported.
Virtualization is a great idea. It beats dual boot arrangement or having multiple machines. VirtualBox has made it very easy and cheap to do. It's great for the occasional or medium duty user.