Since the emergence of the Internet online shopping has rapidly grown and replaced parts of traditional face-to-face shopping in real shops in cities and shopping centres. The role of the sales assistant has been supplemented or even taken over by online information like buyers guides, product reviews or product related discussion groups. For example, Amazon offers its customers the possibility to write product reviews which will be published on the product site. However, a potential buyer is confronted with a similar problem as in physical shops: Can I trust the recommendation of the sales assistant in a physical shop respectively can I trust the recommendations - the product reviews of former buyers published by the Internet shop. So, at Amazon's readers of the product reviews can classify a review as helpful or not. In our paper we analyse if there are relationships between the formal structure of a product review and the degree readers classify a review as helpful. We present the results of a case study on the Germany's Amazon shop and derive "writing rule" for good product reviews out of our analysis.