Ratings are on a scale of 1-10:

1-3 = Crap
4-5 = Not-so-average to average

6 = The cut-off point (what Roger Ebert would refer to as a "marginal thumbs down").

6.5-7 = Recommended, but watch the movie first and then make your decision.
8-9 = Incredible film score, I say buy it!
10! = One of the best ever. If you don't have it, you should be ashamed of yourself!