RPXnow is a great service, allowing you to sign in using most popular account providers (like Facebook, Google Accounts, Yahoo, etc.). But my experience with RPX was not that great: the problem is that you can't SHARE your sign-ups among websites. This was very disappointing when we started to use RPX on your website http://www.ecwid.com. We ended up creating our own single sign-on service, and that was not easy. RPX claims to perform as a proxy between you and OpenId & OAuth & other services, still it lacks the very important "smoothness" of OpenId&friends: you visitors still have to click button in RPX iframe to sign-in your website, even if they have already signed in RPX or any of third-party providers RPX supports.
We had a main website and a Uservoice forum, both using the same RPX account. When you sign in the main website using RPX, and then go to Uservoice, you find yourself not signed in there, until you press that dreaded button.
I understand that RPX is popular because of a "wow" effect, but I hope they will add support for SSO in the future, so that users won't be disappointed any more.