Well, don't take P2P as me downloading music and video files, because that's not what we're doing.
Instead of using a standalone external HDD, we have a backup script that I made that essentially backs up our desktop and other laptop (Windows Machines) My Documents Folder and my iBook's Documents folder onto each other's computers. This way, we have all our data on 3 hard disks should one of them fail.
I also have them burned to a DVD as well, but as some people point out, even IF you verify the data, it's very possible that as time progresses (read: years) the disk will become corrupted.
The best solution is of course 2 redundant external HDDs as some people have posted on the forum where you keep one at work (offsite) and one home where you are constantly keeping the one at home up to date, then every month you switch off, so worst case scenario you're a month back from where you were today if you suffered a catastrophic (fire, flood, etc.) loss. If you're paranoid like me, swap them every week.
But, the reason I do the P2P backup is because I don't feel like dropping 200-300 on external HDD's yet.
Here's an image of what I'm doing:
I'm sharing the same information with other PEERS on my network, rather than having them on a centralized location, like an external hard drive. This results in me using MORE space on my network, but having more redundancy. This is fine as long as you have the space to keep it all. My Mac stuff is saved in a folder on the Win Laptop and the Win Desktop (taking double the space), and the same rule applies to the Laptop and Desktop, respectively.
This is an image of a typical "client-server" backup, where all the data is consolidated in one location. This is fine as long as you have a backup of a backup in case the backup itself fails.