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.
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