I second this warning. I've never used a Raspberry Pi, but I've used netatalk for years on a Linux server, and after I upgraded OS X 10.7, my backups to it would get corrupted once every month or two. I think it's not so bad if you have a wired connection, but wireless backups from a laptop (which can get put to sleep at any moment) just aren't reliable, for me at least. Also, it would constantly index my Time Machine backups over wireless, which takes forever and made my Macbook Air's fan turn on.

I suspect some of this is due to limitations in netatalk's AFP implementation, but I've never compared to a proper Time Capsule