Pretty amazing drop in interface to manage a site. I am building a Speakers Bureau for a non-profit and ended up using this to make that part of the build easier. http://activeadmin.info/
My only fear was bringing in way more than I needed but so far it is okay because of how hookable it is.
I have dealt with other systems that can be a pain to hook so I am still always nervous about that moment when the hook takes more time than if you built it your self.
Activeadmin takes your models and makes the forms needed to manage them on the fly. The one issue I has was a model with no field it could figure to use as the select label. So all I had to do was set my model to have add
alias_attribute :name, :firstname to the model that was having an issue. (not the belongs_to model but the has_many) and it just worked.