Depends how badly I want to know

That sounds like a really cool plot. What's it called?

5 stars: It caused some kind of emotional response. E.g. I cried, or I just loved it so much I hugged it at the end.

4 stars: It was good, I enjoyed it.

3 stars: It was okay. I got through it.

I never rate anything below 3 stars because if a book is that bad I'll put it down before finishing it.