I did OpenID early on in the site (nobody understood it), RPXNow/JanRain (more than we could afford for fewer features than we needed), and finally our own solution that does OAuth* for the various providers and Persona.
I love Persona. I want a Persona shirt and tattoo. OpenID was just a bit harder, but had nowhere near the payout that using Persona offers. The only burden of Persona is the auto-login/logout handling, which is a good thing in the long-run but in the short-run involves tracking down all your login/logout methods and making sure they all react properly.
*OAuth integration is a nasty mess of nastiness that I can't bring up w/o derailing this entire conversation. If you want to chat about that train-wreck, email me.
I also adore Persona. It took me two minutes to integrate on my Django setups the first time, and less than that on subsequent ones. Nowadays, it's all I use.
Not the GP, but I was thrilled with how easy it is to integrate. I have a little site that I'm putting together as a hobby, and it took me about an afternoon. Web development isn't my specialty, so if you know what you're doing you could probably do it in less than an hour.
What's your experience with that? Implementing OpenID tends to not be very fun (whether as a consumer or as a provider), is Persona better?