For what it's worth... I use Netgear SC101 Network Attached Storage units for my backups.

They are a bit 'clunky' and messy to set up with a weird file system called Z-San, but have the advantage of allowing me to install two drives per unit and set them up 'mirrored'.

I've been using them for around four years, now, and have had one unit fail, replaced without question under warranty, and a couple of drives have failed... but on each occasion I didn't lose data thanks to the mirroring, which has to be good.

The daft thing about them is that each unit needs its own IP address, each HDD needs its own IP address, and each partition needs its own IP address! It does mean, though, that the mirrored drives don't have to be on the same unit, which gives additional security - as long as they're on the same network they don't even have to be in the same building!

On the downside they are rather slow when compared with a USB 2 or FireWire external drive. But if you're leaving a backup running overnight, who cares?!