I'm developing an Windows Azure web application and I would like to understand if I should use Windows Azure Control Service (ACS) in my Architecture or not. So, my questions are:
Well "valid" as in it works and is supported, yes! But I'd say the value is more as a way to map identities and claims presented from other identity providers - like Live, Google, etc. - to claims for your application to respond to.
You can certainly set up your own users via the ACS portal, but for a true membership system you want the user to be able to change passwords, profile information, etc. Much of that can be accomplished via the ACS Management REST API but I don't know of any higher level abstractions or framework that you might be able to leverage. It seems like it would be a LOT of work that isn't really a core part of your application.