Although personally i don't think its a great spec. Its a good enough spec (certainly better than saml, shudder) - "good" is not the same as works acceptably or popular.
After extensive experience with SAML and other specs, I don't think it's better than SAML in a fundamental way. It's certainly better in that it doesn't require a mind-numbingly verbose blob of XML, but strip away the XML and all that verbosity, and you basically end up with... kerberos.
