It is important to design a product user interface based on feedback from qualified users. For example, design a journal entry screen based on comments from the general ledger clerks who will be using it regularly. This seemingly simple concept is often overlooked, and UI designs instead are based on 'tests' done on the developer sitting in the next office. Only the actual user of the product is qualified to comment if the UI is appropriate.
Be sure to sample a wide variety of users; feedback from users at a single customer site will be biased toward their specific implementation of the task. Whenever possible get feedback based on actual 'hands-on' testing, rather than just paper prototypes.