I have been using RPX on my site to allow users to login through Facebook and Twitter. This is where a majority of my sign ups come from. It really depends on what type of site you have.

I don't get many return users from it but I do get some extra information which is useful in my case for statistics.

I am currently moving away from RPX in favor of using the services for Facebook and Twitter directly so I can do more with the user and with the data.