I don't think it's reached the point where it should be the only authentication system offered by any service.

I think it's still very confusing to the average joe, but if you stressed something like "login with your google account", with an easy button to use the Google OpenID provider to authenticate, it might make it easier for the average joe to understand. Something similar to http://www.postrank.com/login would probably be effective with most average users, IMO.