The Rating System

Roger Ebert uses stars; 0 - 4 in half star increments. He offers the following:

4 star or 3.5 star reviews... signifies that I believe these films are worth going out of your way to see, or that you might rent them, add them to your Netflix, Blockbuster or TiVo queues, or if they are telecast record them.
Zero Stars, One-half Star, One Star, and One-and-a-half Stars. These are, generally speaking to be avoided. Sometimes I hear from readers who confess they are in the mood to watch a really bad movie on some form of video. If you are sincere, be sure to know what you're getting: A really bad movie.

Empire has stars from 1-5. They provide the following guidance:

STAR RATINGS EXPLAINED
Classic
Excellent
Good
Fair
Tragic

IMDB allows a rating from 1-10 and shows an average to 1 decimal point. By way of a guideline the top rated film (Shawshank Redemption) scores 9.2. The films ranked from 192 to 250 (and beyond) have an average score of 8.0. The bottom ranked film (Superbabies: Bay Geniuses 2) has an average score of 1.4!

We agreed upon a 5 star system and offer the following descriptors:

5 stars - Unmissable
4 stars - Excellent
3 stars - Good
2 stars - Underwhelming
1 star - Painful