The use of mockup for specifying graphical applications

Specifying an application is the corner stone of software development success ; well performed, the end-product has a chance to be compliant with the customer needs, if not, the success is unpredictable.
We've seen in a previous post that the specification phase consists in defining the problem domain, and especially a knowledge model in interaction with the system under study. So we have to focus on formalizing the definitions, the domain rules and the functionnalities the customer need.
Many technics may be used to perform theses tasks, and especially use case diagrams, sequence diagrams, user stories, and so on...
Once we begin to have a good understanding of the problem, we have to ensure that this understanding is in conformance with the customer one. This has to be done before starting any development of course.
The use of a mockup is then highly recommended, especially when designing a graphical application, but obviously not only.
There are many ways to do this :

• word documents or diagrams on paper : it is the old way. It is very uneffective, especially when there are many stakeholders in the decision process
• diagramming tools like visio, smartdraw, ... : it is better than nothing, but we have to face to a static way of presenting the mockup... The dynamic aspect is bypassed... and yet, ergonomics is also a matter of dynamics
• graphics tools like photoshop, paintshoppro, .. : the result is smarter than with the former tools, but the result is the same. We may also introduce confusion while mixing application design, and presentation design, which are two different things in my opinion. We also need some specialists to perform this task.
  
• development tools like dreamweaver, eclipse, visual studio : in that case we can have dynamics and then have relevant advices from the customers. The counterpart is a development time which is not compliant with the iterative and quick process of specification. By experience, it is a very consuming task. The use of GWT or similar frameworks may nevertheless be taken into account to reduce the drawback. But the collaborative aspect of the specification is absent.
  
• wikis like google sites or editme : it is an alternative way of performing the mockup ; it is quite easy to use, it is a collaborative (a major point, we may for instance leave comments on each page), quick to develop. But up to know, we still lack of ready to use components/wireframes to simplify the development. Another alternative is to use powerpoint to do this, but it is less collaborative.
• specialized tools like Axure ones : the goal of this kind of tools is to help the specifier to quickyl develop mockup by combining the advantages of all the former solutions described here. Of course it is not gratis, but I think this approach is the right one.

As said previously, I thing we have to dissociate the functional mockup to the graphical design.
The use of wireframes can be very helpful to do this (another link).