Product reviews are a type of user-generated content that can be beneficial for both customers and product designers. Without quantifying the design knowledge present in product reviews, however, it is hard for the companies to integrate reviews into the design process. Several studies investigated review helpfulness in general, but few works explored the problem of review quality from the perspective of product designers. In this study, a theoretical model is presented and a system is proposed to assess the quality of online product reviews from the viewpoint of product designers. The system involves an original similarity-based metric to quantify the design information content of reviews on a continuous scale. Experiments are performed on a large number of digital camera reviews, with results indicating that the proposed system is capable of recognizing high-quality content, and would potentially assist companies in product improvement and innovation.