And how much have I not had to bill my clients for the incredible functionality of cancan (and to a lesser degree nested_form). cancan (https://github.com/ryanb/cancan) is probably the most valuable gem I install in every Rails project.

I combo devise+cancan+carrierwave+activescaffold => insta-webapps. The activescaffold cancan bridge I wrote chains the cancan rules throughout the app, epic instant gratification.