Everyone is posting their rating system. Here is mine.
5 stars - love it! Probably will reread it one day.
4.5 stars - very good. Definitely will reread it. Must own. My bookshelves are full of 4.5 star books.
4 stars - good. I enjoyed it, although it has some minor problems.
3.5 stars - practically the same as above but with variations due to my personal taste.
3 stars - OK. Read and enjoyed but not 100%. A solid reading fare.
2 stars, 1 star - I seldom give those. It means I dislike the book, and if I dislike it I usually DNF it. I almost never post my DNFs. Too often, they reflect not the quality of the book but my personal opinion, and I don't want to lower the rating of any book by a living writer just because I don't like it. Someone else might.