Since opinions differ on ratings and what exactly certain numbers mean, I thought I'd just write a summary of how I see it.
My ratings system is based on a 1 - 10 scale. Basically, anything on the low end (1 - 4) is bad, in the middle (5-6) is ok, and on the high end (7 - 10) is good.
The following is a more detailed explanation of the specific ratings:
10 - Outstanding. A book that stuck with me long after I put it down. The only books I would give a 10 rating would be books that made it into my list of favorite books ever.
9 - Excellent. The book was very good, but was better than your average good book by being unique or thoughtful, having great characterization, and/or just being a lot of fun to read.
8 - Very good. Not a book that kept me thinking about it a lot after I put it down, but very enjoyable and better than just a worthwhile book.
7 - Good. Worth reading but nothing spectacular or mind-blowing.
6 - All right. An ok book that had some interesting elements.
5 - Meh. It's not terrible but nothing about it intrigued me for a second.
4 - Somewhat bad. Definitely not worth reading.
3 - Very bad. Not worth glancing at.
2 - Terrible. All memory of this book should be erased.
1 - Complete crap. The author must have had incriminating photos of somebody in the publishing industry. Books like this are almost enough to make one believe in book burnings.