Retrospect has a "duplicate" command in addition to the "Backup" command. I've never had a "duplicate" fail to be a totally viable mirror-image of the target volume. You can boot from the backup immediately, unmount your internal HD (or otherwise original volume, it's usually an internal HD), and test it if you wish.

I use Superduper, and it runs for me at 4am every morning. About the only thing it doesn't do, that Retrospect does do, is historical backups; meaning, make a copy of what your Mac looked like on Monday, then Tuesday, then Wed, etc. So that you have more than just yesterday's backup.

Retrospect is the best, in my opinion, but its interface was always too confusing for me. I could never tell if I was doing what I thought I was doing. If they ever hire a GUI expert and redo its interface, I'll jump back to them.