I also use duplicity for my backups. I don't use the S3 option, preferring to write to my own storage, but if I were to turn on S3, what exactly does Tarsnap give me beyond what I'm already getting? Other than an additional middleman taking a cut?
Duplicity isn't bad; I often recommend it to people who want something similar to Tarsnap but don't want Tarsnap itself.
The biggest disadvantage to Duplicity is that it has a far more constrained archive management model -- with tarsnap you can delete any archive at any time, but Duplicity works with a "full plus incrementals" model, which means that (a) you can't delete an archive without deleting the archives which "depend" on it, and (b) you'll inevitably need multiple full archives.
Other points which are probably of lower importance to most users: Duplicity's website and documentation is even worse than Tarsnap's; and they rely on GPG, which has a pretty lousy security track record.
