MedioVis is a visual information seeking system which was designed especially for library data. The objective target was to create a system which simplifies and optimizes the user's information seeking process and thus further motivates the user to browse in the library stock. To enhance the motivation special attention was given to consider joy of use aspects during the design of the user interface. The primary user interface design is based on multiple coordinated views to offer a great variety of exploration possibilities in a direct-manipulative manner. To accomplish a self-explanatory usability of the system for non-expert users, the development was accompanied by continuous user tests with casual and regular library users. At the end of the development process a comprehensive summative evaluation was conducted, comparing efficiency and joy of use of the existing web-based catalogue system KOALA of the library of the University of Konstanz with the MedioVis system. The results of this comparative evaluation show a significant improvement of the efficiency of the information seeking process with the help of MedioVis. The users also rated MedioVis significantly better in all dimensions of its hedonic quality and appeal compared with KOALA.