We've been live for several months now in the Real World - our userbase (amateur athletes) is primarily nontechnical. About half of our users choose Persona/BrowserID and half choose Facebook. We were initially concerned about the BID login flow (in particular, the immediate email roundtrip) but it hasn't been a problem and the UX has been refined quite a lot over the last month or two.
For a mass-consumer audience, the combined FB/Persona solution is excellent:
* Facebook unquestionably has the slickest auth experience, even eliminating the followup name/sex/bday questions. However, a significant percentage of the world (possibly > 25%) either Hates Facebook or wants to keep their Facebook account isolated. This is unlikely to change in the near future and could even get worse depending on what sleeping dogs Zuckerberg decides to kick next week. We don't have the option of alienating the FB haters and we wouldn't want to anyways.
* The Persona UX is good and rapidly getting better. BigTent integration with gmail, yahoo, hotmail will bring one-click login to those users. A native experience is being built into browser chrome. All this is coming without me having to write code. It may not be as slick as Facebook, but I like where this train is headed.
* Integration is simple compared to writing a username/password system. The API is incredibly easy to work with. Dual-auth with Facebook is a little more complicated, but a complete Persona-based auth system is a question of hours, not days.
* The fact that identity is just an email address makes it easier to integrate with existing login systems. In our system, you can log into the same account with both Facebook and Persona as long as the emails match. No, email is not a perfect identifier, but even nontechnical users understand it immediately and really - what other option is there? "What email address did I use?" is a lot better than "What weird combination of letters and numbers did I use as a login name?"
* Support on the Mozilla dev-identity list has been fantastic.
We're pretty happy. Honestly, I don't ever see myself writing another username/password login system ever again. Persona is less work for a better UX.