By testing in-house, you run the risk that you will offer testers leading questions, which end up tainting the results. Think of it like plumbing or fixing your car. In most cases, it's best not to take the job on yourself. Instead, get an outside source to do the work for you. Regular users are often confused if a site's navigational style changes and should be queried for feedback prior to the launch of a new site design.
Regular users are extremely valuable to usability testing.
They can offer unique feedback about a site's functionality and use that you never knew. Groups of regular users often have a style of browsing a site that may be completely alien to the way you intended the site to be used. Usability testing is both a mad science and mastery of the obvious. It also involves logic and an unbiased approach to a Web site. The feedback you obtain can determine the success of your site.