Data from Online Social Networks (OSNs) are providing analysts with an unprecedented access to public opinion on elections, news, movies, etc. However, caution must be taken to determine whether and how much of the opinion extracted from OSN user data is indeed reflective of the opinion of the larger online population. In this work we study this issue in the context of movie reviews on Twitter and compare the opinion of Twitter users with that of IMDb and Rotten Tomatoes. We introduce metrics to quantify how Twitter users can be characteristically different from general users, both in their rating and their relative preference for Oscar-nominated and non-nominated movies. We also investigate whether such data can truly predict a movie's box-office success.