A review may be either formal or informal, depending on the context. Its goal is to evaluate a work, which implies that the reviewer's personal opinion plays a significant role in the process. However, a certain objective standard needs to be maintained and, as in a persuasive essay, your assertions need to be proved.
The formality of the review will be determined by how much of the essay is analysis, how much is summary, and how much is your reaction to the work you are reviewing. A more formal review will not only discuss the work on its own merits but also place it in context. Newspapers and popular magazines tend to review in terms of finance: is this record or film worth spending money on? Critical journals will attempt to determine whether a new novel or play has achieved something new and significant. A good review will discuss both the qualities and the importance of a given work.