Most cities in Germany regularly publish a booklet called the Mietspiegel. It basically contains a verbal description of an expert system. It allows the calculation of the estimated fair rent for a flat. By hand, one may need a weekend to do this task. With our computerized version, the Munich Rent Advisor, the user just fills in a form in a few minutes, and the rent is calculated immediately. We also extended the functionality and applicability of the Mietspiegel so that the user need not answer all questions on the form. The key to computing with partial information using high-level programming was to use constraint logic programming. We rely on the Internet, and more specifically the World Wide Web, to provide this service to a broad user group, the citizens of Munich and the people who are planning to move to Munich. To process the answers from the questionnaire and return its result, we wrote a small simple stable special-purpose web server directly in ECLiPSe. More than 10,000 people have used our service in the last three years. This article describes the experiences in implementing and using the Munich Rent Advisor. Our results suggest that logic programming with constraints can be an important ingredient in intelligent internet systems.