Two or three comments noted that Unit Testing is no substitute for User Acceptance Testing and that both are equally important.
They missed the point entirely. Both are worthless if the interaction design sucks. (Any Lotus Notes user can testify to that - http://lotusnotessucks.4t.com/index.html)
And because Interaction Designers are so rare in the wild, every developer that develops any kind of user interface must strive to become a better interaction designer, knowing that they represent a conflict of interests. And every Business Analyst the designs user interfaces has all the more responcibility to excel at great interaction design.