In recent years many system dynamics modelers have pointed out that for effective implementation of model results it is that the client participates in the model-building process. This has lead to various more or less successful approaches in group model-building. However, up to now little systematic research has been conducted in the area of effectiveness of group model-building. Systematic evaluation of group model-building is important in order to a) understand how clients and organizations are effected by group model-building; and b) improve the effectiveness of the group model-building proces. In this paper evaluation results are presented of four model-building projects based on clients' opinions of the successfulness of these projects.