Updated 3-24-14 to add one caveat: These numbers from 2008 are all based on how much I personally enjoyed the book. I can't judge for anyone else, so my ratings are completely subjective. As a matter of fact, if I re-read a book, my opinion may very well change, based on what's going on in my life at that point.
10 ~ Loved it!! Couldn't put it down!!
9 ~ Excellent!
8 ~ Very Good
7 ~ Good
6 ~ Above Average
5 ~ Average
4 ~ Struggled to finish, but not worth it
3 ~ Annoying ~ a waste of time
2 ~ Poor ~ one I abandoned
1 - Pitiful!
0 ~ Awful!! Don't bother
* DNF ~ Did Not Finish ~ discarded the book as a total waste of time
Updated 3-29-19: For a few years I tried eliminating anything below five (5/10) by using "Nah" as a rating. I tried "Not My Cuppa" (with "tea" implied) a few times. Now I'm back to my original numbers because I just finished a book that needs that three (3/10) rating.