Including usability in the design process is often considered expencive. Usability testing doesn’t have to be expencive, though. With only 3 to 5 users you can find majority of your application’s problems.
At first, do a pilot test to make sure your task descriptions are understandable to others. Remember to include test users from every user group to your actual tests. That way you will get a wider perspective for your results. The test criteria may vary between different user groups. For example, novice users may be given a longer time to complete some tasks than expert users.
What do you gain from usability tests, then? You can see if there are features that the users can’t find. You will find out if they can understand the terminology used, or if it takes too much time to find the right keys or features? Most importantly you will find out if the users can actually complete the tasks they are asked to do and if they are satisfied with the experience of using the application. Usability testing improves your application’s quality!
Also read Jakob Nielsen’s article Why You Only Need to Test With 5 Users.