I'm ready to go out and celebrate Earth Day, thanks to Carbon Copy Cloner. A few days ago, prompted by Software Update, I had done a routine upgrade from Mac OS X 10.3.8 to Mac OS X 10.3.9. Today I discovered (via trial and error) that a proprietary application I rely on for my work had not yet been upgraded to be compatible with 10.3.9. After freaking out for an hour or so, I remembered that the most recent bootable backup of my system on my Firewire hard drive was in 10.3.8. So I simply switched the preference for my startup drive, restarted, opened the proprietary application in the old OS, and life was back to normal.
This is the third or fourth time the La Cie external Firewire hard drive and Carbon Copy Cloner backups have saved my bacon. The drive was about $150, and CCC is free. Highly recommended.