For me, here's how it goes:
5 = I loved it so much, I would read it again
4 = I liked/loved it and would recommend it, but one reading is enough
3 = I liked most of it and would probably recommend it to a certain type of reader
2 = it was barely good enough to finish, and I wouldn't recommend it to anyone I know
1 = it was bad, I couldn't finish it, and I wish I hadn't wasted my time trying
