I use the following rating system:
1 star = I disliked the book
2 stars = The book is OK
3 stars = The book is good
4 stars = The book is really good
5 stars = The book is amazing
If I give a book three stars that means I think it IS worth reading.
I do not give half stars. My rating just gives how I personally reacted to the book; maybe you will react differently. I do try to explain WHY I have reacted as I have. Hopefully this will help you decide if a book I liked or disliked will work for you.
With audiobooks I rate the book, the written lines. Although I do also state what I thought of the narration, I do not take this into consideration in the rating.
If you want to see my reviews, search for the tag chrissiereviews