5 star rating systems are obnoxious. From a mathematical perspective, if you treat them in an ordinal fashion they are poorly behaved, and if you treat them categorically, you lose the relationship between stars. There seems to be some popular movement towards binary rating systems, and I think that is great. Not only do people tend towards binary rating behavior in the real world (only rating a movie they thought was very good or very bad) but they admit a much cleaner mathematical treatment.