Persona seemed like a pretty good system, until it was shut down. I don't like single-sign-on systems in general, but having it managed by the Mozilla foundation seemed a lot safer than relying on Google/Facebook. I think their original long term plan was to transition users away from their single-sign-on service as browser support for the features they needed got better.
The only thing that bothered me about the general design is that it used email addresses as identification tokens, and for the site I was making I didn't even want that much personal information from my users.
