It's also great for testing clusters on a single VM. I've been using it for playing around with different mongodb configurations. Being able to instantly start, stop, and fork server images is pretty amazing. Also very nice being able to replay commands from the cache, so downloading and building a bunch of packages only has to work once. It doesn't do anything that you couldn't do with virtual machines or ec2 images, it's just so much faster and lighter weight it's kind of like the difference between having to compile your code after every change and having live reload. Much better for your productivity.