I did some work for a major telco and we weren't allowed to do user testing outside of the company courtyard. It consistently blew my mind how "out there" the

I did some work for a major telco and we weren't allowed to do user testing outside of the company courtyard. It consistently blew my mind how "out there" the feedback was with about a half-a-day-worth of testing. So I don't know about 5-users, but I can say having about 25-50 is good for getting a broad sample. If you get the wrong 5 users... you're going to get some really skewed results. For example, if you grab a group of 5 people and all of them are in tech your results are going to be dramatically different than if they are 5 people from accounting, or 5 people from janitorial services.