The paper describes a collaborative Mixed-Reality (MR) environment to support the product design assessment. In particular, we have developed a collaborative platform that enables us to improve the design and the evaluation of cars interior. The platform consists of two different systems: the 3D Haptic Modeler (3DHM) and the Mixed Reality Seating Buck (MRSB). The 3DHM is a workbench that allows us to modify the 3D model of a car dashboard by using a haptic device, while the MRSB is a configurable structure that enables us to simulate different driving seats. The two systems allow the collaboration among designers, engineers and end users in order to get, as final result, a concept design of the product that satisfies both design constraints and end users’ preferences. The usability of our collaborative MR environment has been evaluated by means of some testing sessions, based on two different case studies, with the involvement of users.