Questions about authorization are often poorly formulated because we are all interested in different aspects.
In my case, we have a well developed application (Wagn, see http://wagn.org), and it has a home grown authentication functions built in already. What we are doing is making the application independent of the authentication system, so I am most interested in two dimensions:
1) What is the API for the provider interface and how easy is it to add to my app.
I've just done a lot of work to route all of it through a set of class or module methods that the application uses, and a model for the home grown AR class (a User class and users table) that. This leads to the next part:
2) What authorization providers are available and what do I have to do to load them with my application (typically I would expect these to be in one or more Rack middlewares).
I don't need Devise, it does too much, I want an interface to external auth providers. My app may provide forms that will post parameters to the auth services, but it won't be using their Rails views or controller.
I suspect this situation is common for a certain class of developers.