I use rdiff-backup as my main backup solution. It's efficient and restoring from the latest backup is reasonably fast and simple. But I've found that the further you go back in time, reconstructing the file from all the deltas takes so long I don't consider it useful as an archiving solution. At least that seems to be true for large and/or complex collections of files. Have you actually restored large databases from years ago in less than a few hours?