I swear by SuperDuper. I'm a computer consultant/trainer that supports several different versions of three different software suites.

While the files are backwards compatible, the programs are not and have to be run a separate, discrete installs to work properly. SuperDuper lets me do that easily, with four external disk drives that let me selectively install the appropriate version of the software I need to support on my laptop, then safely blow it away when I'm done and return to current versions of my software configurations.

If only I could find something that worked half as well on the Windows/PC side, I'd be an even happier camper ...