At Smartupz we are trying to do the lean model and customer development. It's much harder then it looks - first problem is that people you are trying to approach (in our case small/medium business) have no time. So you must be very clear in terms of your value proposition so they can immediately "get" your app and share their opinion.

Creating crisp/short value proposition statement is damn hard.

Second thing - throwing at people MVP or just a presentation with the proposed functionality works fantastic! We get tons of great feedback without actually having to write a single line of code. When we eventually start writing code - functionality is much more "down to earth" and customer is willing to test it because it's partially his baby.

cheers, Jarek.