After a while we started incorporating the final UI framework in the prototype. It made the prototype look and feel almost like the final product. We got very good clear feedback from customers and incorporated their feedback into the prototype almost everyday. @enricgili and I built the prototypes ourselves without having to rely on another dedicated prototyping person.
The prototype is the specification. We did not write very elaborate requirements documents or use cases.
We are still developing the product. So we may still run into usability related issues and minor feature issues. But we are pretty clear about what customers are willing to buy and use to make them better companies. The process is not perfect. I am not sure if it is scalable or even repeatable. But it works very well for us even in a large company such as SAP.
@MChewD and his team are trying to take this one step ahead and write code that customers can try. It is not easy to do this within SAP. But they are doing some innovative things.