One of the principles of Agile Development (via the Manifesto - http://agilemanifesto.org/principles.html) is to deliver working software frequently. One of the main ways we do this within the OSS development group is to get screens out there to our users early. Even if the screens don't do a whole lot, the functionality that they do contain has been tested and works.
Throughout the development of an application we're continuously pushing out features in small iterations to our customers. Even though the product is not fully "baked" it allows them to see visual progress, begin working with the product, and even provide testing and feedback.
So far it has produced great results. Our customers know exactly what they're getting, they've had interactive collaboration with us throughout the development effort, and the end result is a custom technology solution that meets the business need.