I use a five star system, with five stars being the highest accolade I can give. Below is a more detailed description of my rating system.
Five Stars means I loved it. I enjoyed it tremendously and I would recommend it to everyone. It does not have to be a literary masterpiece but it managed to draw me in.
Four Stars means I enjoyed it. I would buy this book and enjoy re-reading it but there is something missing that means it does not quite get in the five star category.
Three Stars means I liked it. However I was not blown away by the story. This is a solid rating and not to be sniffed at.
Two Stars means that I did not particularly enjoy the book, but it held enough interest that I made it to the end.
One Star is a rating that (hopefully!) will never be used. It means that I never finished the book, and the book is most likely off to the charity shop.