My rating system is pretty simple: 1-5 stars; based on the system from Visual Bookshelf where I first started rating books. I sometimes assign half stars when I can't quite decide on one category or another.
One Star: The book was just plain, stinking awful. If I finished it, I'm not sure why and there's a good chance that I didn't. If I bought it, it's leaving the house. If it was borrowed, it's going back where it came from ASAP.
Two Stars: It wasn't horrible but could have been better. There are a few redeeming qualities--maybe I like the characters or the plot, but it doesn't all come together.
Three Stars: This was a decent outing. Enjoyable read, no real complaints--but nothing really knocked my socks off.
Four Stars: Over-all pretty outstanding. Lots to like from characters to plot to style to setting. A book I'd probably read again (if there weren't so many on my TBR list already) and would recommend to others.
Five Stars: A must have, must read, must recommend book. Absolutely knocked my socks off on all counts. Even though I really don't have time to re-read books, I will re-read this one (probably more than once). I may bore my friends to death talking about how great this book is and will recommend it to everyone whether it's their kind of book or not. A book to obsess over.