The User service is handy and simple. That
it only handles Google accounts is a problem for me. I've created a
lightweight identity library (LID) which is similar to the User service
and provides authentication for Google, Facebook and any Email.
Google auth is via the User service, Facebook via OAuth and Email via
an Email sent to the user which contains a URL they visit to sign in.
Sign-in is cookie based.
Its not general purpose, requiring Guice and running only on App
Engine but it may be useful for someone if the User service is
functionally OK but doesn't provide enough coverage.
Tim