UXmatters have a well-argued discussion of the pros and cons of checking your design by user testing, as opposed to having an expert do a review. These methods achieve different things. For example, suppose you have a design including green navigation tabs, with a red colour being used to show highlighting. A usability reviewer should immediately point out to you that this design is not usable to anyone who is red-green colourblind, which is a point you might miss if you tested with real users, and none of them happened to be colourblind. On the other hand, expert reviewers can suffer from their own biases about how things ought to be done. The ideal is to do both - review the design from a usability point of view yourself (bringing in an expert if needed) and then try it on real customers. Real customers are the only way to get the authentic - and sometimes unexpected - voice of the customer. Among the things real users can do for you is to help you explore whether you've designed workflows in the way that the users (or most of them) expect.
Thanks to @IATV, whose tweet alerted me to this UXmatters article.