I try to give star ratings according to the genre, the context of the time it was made, and the subject matter being tackled.
A movie tackling really big themes and making bold steps deserves a reprieve from a few blunders, whilst a formulaic comedy really needs some perfect footwork to be worth watching.
At the same time, some genres really can't get a whole lot better. No Blaxploitation can really be called cinema, but for their context in cinema history, are given fair ratings.
That said, the 4 and 5 star films manage to transcend their genres limitations (An action film with a good plot, a horror film with a good acting, a comedy with character development, etc.) 3 star films are average, they fall in line with their genres expectations, whilst 1 and 2 star movies might give you a pretty poor impression of the entire genre itself, and leave a bad taste in your mouth even for good movies of a similar theme. That isn't to say the movie itself is bad, it merely shows the rather seedy, unsuccessful side of a genre. Plan 9 From Outer Space is a film worth watching, but because it represents the same Sci-Fi genre as "Alien" or "The Day The Earth Stood Still." Poor films are necessary to appreciate the good ones.
Finally, entertainment value is something I really can't rate, but I'll note in a review. I really enjoy some pretty terrible movies, but I'll star them for how they are crafted and executed, and only rarely will I favor or deride a movie for its sheer entertainment value alone, or simply because I dislike it's content. When I do, it'll be obvious.