Riccardo Mori (via MacStories):
Remember: hard drives die unexpectedly in most cases. (Solid State drives too, in case you’re wondering). It will happen when least you expect it. It will happen at an inconvenient time. You will be bothered. If you don’t have a backup of your stuff, you will also be panicking. Be prepared.
If you have information on your computer that is worth more than a hard drive (in both monetary and time-to-recreate terms), and you’re not backing up, you’re nuts. Time Machine is the most straightforward (and cheapest) approach, but it isn’t bootable. If you’re cool with that, it will probably work fine.
If you have some sort of crazy multi-drive setup, or you need your backup drive to be bootable, SuperDuper! is a delightful choice.
If you’re looking for a backup hard drive, I recommend something very quiet and unobtrusive, so you can leave it on all the time. I have a Buffalo MiniStation — it’s bus-powered so it’s convenient, and it uses Thunderbolt, so everything is instantaneous. If you’re feeling totally baller, pick up a solid state external drive.