I am in my first steps of implementing OpenID in my Rails app. open_id_authentication appeared to be a fairly easy-to-use plugin, which is why I decided to use it.
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this post contains a good strategy to use AX for google and Sreg for others, to make this happen a little more seamlessly http://www.franzens.org/2009/01/using-google-federated-login-in-your.html
I've also stitched together a complete solution to Ruby on Rails 3, OpenID, and Google: http://blog.sethladd.com/2010/09/ruby-rails-openid-and-google.html
The authenticate_with_open_id return the Sreg object, not the AX response. So you need instanciate this respone with Rack::OpenID::REPONSE like that :
ax_response = OpenID::AX::FetchResponse.from_success_response(request.env[Rack::OpenID::RESPONSE])
After you can fetch your data
ax_response['http://axschema.org/contact/email']
ax_response['http://axschema.org/namePerson/first']
ax_response['http://axschema.org/namePerson/last']