I completely agree with your last point on getting feedback and iterating on your designs. Prototyping is such an important part of the design process. ABP (always be prototyping.) I wanted a prototyping solution that worked within a designer's existing workflow which is why we built Stand In [http://standin.io] on top of Photoshop. It allows us to keep the prototype always in sync with the design and truly becomes part of the design process — not just a step afterwards.