Writing a review is an evaluation of something. Writing a review puts you in a privileged position - your readers really want to know what you think about something. Whether they decide to read the book, watch the film, or buy the product is dependent on the overall impression you give. To write a review well, you need to sound like someone people can trust - you only have to read some of the badly-written book reviews on some websites to see how important the writer's tone is.