Margaret Slusser mentioned several interesting points. The survey instrument must be well designed to avoid an unexpected response. Users may not answer honestly if they think their responses will change the service that they need. It is also important to decide what you are measuring before you begin the survey process. Although we have had a slow discussion so far, I would like to add another element to the question.
Many people consider themselves to be active library users yet never enter the building.
I hope some people will be returning from vacations this week to join the discussion. This message may bring two discussion groups together who share similar interests. As manager of an eLib project with a major user needs element, I wish to emphasize the following factor which controls for bias in surveys. Random selection of the population from which you expect end-users to come from is essential. If you only contact persons who use your product or service then you will never know what those who might use it want and need.