The author really should have a backup plan in place. He still doesn't, even though this software lets him sleep easy at night. He's still riding the edge of disaster if a file ever becomes corrupted and then synced. Ut oh. Incremental backups, folks. It's really not that hard.
On the open source side of things, Dropbox and BT Sync aren't really that interesting. You could whip up similar functionality with a bash script, inotify, and rsync in a matter of hours. Dropbox is only relevant because they give you off-site always online dumb storage. There will never be an open source or "trustworthy" solution to a cloud storage service. This Raspberry Pi idea is pretty neat, though. But local encryption is always the answer.
Also, if you have important data that should be encrypted, then it's a pretty stupid thing to have it synced all over the place. You don't want every device acting as a possible point of compromise. Principle of least privilege, and all that.