Ratings are given on a four-star scale and apply to the DVD only, not the movie itself. Keep in mind that these ratings apply primarily to import DVDs, which on the whole don't have as good quality as US DVDs. A "very good" Hong Kong DVD may only be an average US one. Prices quoted are US retail; prices from direct importers are often lower.
1 star = Poor picture quality (blurry, choppy, grainy), usually not letterboxed and/or subtitled, bad sound, little or no extras. Avoid.
2 stars = Average (good VHS) picture quality and sound mix, letterboxed/subtitled, decent extras (trailers, biographies/filmographies). Worth a rental.
3 stars = Good (better than VHS) picture quality and sound mix (good channel separation and bass response), bonus extras like music videos/featurettes. If you like the film, it's worth buying.
4 stars = Excellent picture quality/sound mix (much better than VHS) and tons of features like deleted scenes and commentary. Worth seeking out and buying, even if you already have it in another medium.
Note: some movie reviews also have information about DVDs