Remember the concept of "porn buddy" as introduced on the UK show Coupling? You exchange keys with your most trusted friend, so in the event that you die an unexpected death, he (or she) would come over and take away all your porn so your family won't find it when going through your stuff. This backup service Cucku is similar to that, except instead of taking away your porn when you die, your porn buddy keeps it safe for you while you're still alive. Which is probably even better.
Cucku calls it "social backup", but it's just a program that transfers your data onto your friend's computer for safe keeping, and vice versa. Peer to peer backup would be as appropriate. Your backup is encrypted, so your partner can't go poking around your stuff, but still offers the reliability and safety of offsite storage.
We can see a few problems with this setup. Their estimate is that 500MB takes 4 hours to back up on a normal ADSL connection. This is fine if you only have 500MB, but if you want to image your entire drive—500GB or so—that'll take 4000 hours. That's 166 DAYS, or about five and a half MONTHS. This is why online backup solutions need an initial sneakernet transfer of hard drives loaded up with your data (which Cucku does not offer in its current implementation). Everything is fine when you're just doing incremental backups after the fact. Also, you're limited to how much storage space your friend has, so you'll probably need to buy him a new hard drive as well.
Is it a good idea? Sure, but only for your most valuable files, and only if they're not more than a few gigabytes in size. [Cucku]