SuperDuper! is a backup app designed for one particular type of backing up: cloning.
The idea is that you make a complete duplicate of your Mac's hard disk so that in the event of your internal hard disk failing or becoming corrupt, you simply hook up your external, backed-up-to drive and boot from that, enjoying essentially zero down-time.
If you've swapped the broken internal drive for a new one in the interim, simply clone from the backup to the fresh drive and you're done. (With Time Machine you'd have to boot from your Leopard DVD and do a lengthy system restore.)
SuperDuper! delivers on ease of use and security. Plain English text makes sure you know what's going to happen when you press the button.
Leopard compatibility in 2.5 means not only that it plays nice with 10.5 but also that it understands Time Machine; SuperDuper! can store system clones on the same partition as your Time Machine backups when you use its Smart Update feature.
We did have a few problems with this approach, though, so we still recommend a dedicated partition or volume for your cloned drive.
Simple but not very flexible
Backups can be scheduled - though the interface isn't as powerful as rival app Carbon Copy Cloner's (http://bombich.com), and it's less simple to exclude particular files or folders - and Smart Backup only updates changed files.
The developer allows you to use the app for free, but you don't get Smart Backup; you have to do a full clone each time and it wouldn't protect Time Machine backups on the target partition.
Whether you use SuperDuper! or Carbon Copy Cloner comes down to what kind of user you are. SuperDuper! is simpler but not as flexible.
For our money, though, we think its balance is a bit more usable than CCC's so it gets our highest accolade.