SystemImager tutorial: Imaging your Linux system

Have you ever had the problem that you have set up the "perfect system", and now you want to back up this system before you make changes to it so that you can restore the original state if you changes are not satisfying? Or are you a system administrator in a large company where you have to maintain hundreds of Linux machines that run exactly the same software, but are sick of installing each machine manually? Or did you develop a Linux-based hardware appliance, and now you want to sell it in big numbers on different hardware platforms (i.e., different hard disks, etc., not different processor architectures!) without having to maintain an image for each platform?