The current rating scheme is a simple five-point system: 5 (Excellent), 4 (Good), 3 (OK), 2 (Poor), and 1 (Bad). If it helps, there can be a virtual 6 (Perfect) and 0 (Unredeemable).
The images used for each are:
Excellent- An excellent experience, one of the best of its genre.
Good- A good experience, however flaws prevent it from receiving the highest ranking.
OK- While not accessible to all, something ranked OK will likely appeal to people in its niche.
Poor- While not completely worthless, severe flaws prevent it from scoring higher.
Bad- The worst, not worth having for any reason.