the "elevator pitch": a means for us to gather feedback on the design of the system in the context of specific real world tasks. By asking users to use the system (or a reasonable facsimile) we can observe opportunities to improve the design, catching them at this stage of the design process rather than later when changes would be more costly.(Brown)
In essence, Usability isn't asking users what they want, it's seeing what they need.
The process is relatively simple, you isolate your user groups, you establish a task list for them to perform, and then you carry it out. . The concern with doing it yourself would be lacking the ability to take an unbiased look at the thing you have created (