I've been beta testing SuperDuper! almost since the beginning, and it has become indispensable to me. It's easy to use out of the box, but it's also highly customizable so power-users can really do what they want.

For a developer, for exemple, it's very handy to keep different versions of the system as ASR images. It takes less than 10 minutes to restore such an image on a test partition and reboot, and you're ready to test your app.