My current project is in Rails. Coming from a Symfony (PHP) and Django (Python) background, they both have excellent admin generators. Seems like this is missing in Rails.
ActiveScaffold is by far and away the most configurable/easiest to integrate/most automagic scaffolding around at the moment. It has built in ajax support, near seamless db introspection and it even plays nicely with legacy Oracle databases (which can be a real pain in Rails).
Try it: http://activescaffold.com/