Agreed. Switching authentication gems is incredibly painful. Sometimes I think the best solution is to use the "low-level" gems like warden and always craft an authentication system myself.
That reminds me, which is the auth gem of the week currently, Devise?
Devise with omniauth if you want multiple logins
for large values of "week", the railscast for devise is more than year old, and it had already gained quite a lot of mind share by then
Devise is excellent. Jose Valim's gems tend to become standards pretty quickly.