I generally use Goodreads' five star system. I love it.
1 Star: Didn't like it.
2 Stars: Thought it was okay.
3 Stars: Liked it.
4 Stars: Really liked it.
5 Stars: Loved it.
I like the simplicity of it and the fact that it is clearly tied to my feelings on the book. I have always hated trying to assign star ratings to things when they are book-based. Is something a good book or a great book? I have absolutely no idea, honestly. I am not sure I am qualified to judge that or what the specifications are for it. I have certainly disagreed with things popularly considered to be "great works" before, and I am sure I will again.
I do know if liked or didn't like a book, though. This is sometimes due to things that make books good versus great, but honestly, not always. If you lined up all my five star books in a row and said that they were all equally "great" books in a strictly objective sense, even I would laugh.
Although I know many deeply appreciate the half-star ratings here on Booklikes, I have yet to use them and I don't really see myself doing it. It clutters things in my head and makes it much harder for me to evaluate books. I am comfortable with the dividing line I have between "liked it" and "really liked it." I am not sure where to place a line for "liked it and a half but didn't really like it." I liked some of my three-star-rated books more than some of the others, but I am confident in declaring that I liked all of them more than my two-star-rated and less than my four-star-rated.
My average rating over on Goodreads for everything I have read is 3.05, and I figure that is pretty good. Three is my default rating for anything I read and didn't have any serious complaints about. I liked it. And I feel like I "like" most of what I read. I am not seriously annoyed, but I am rarely delirious with joy, either. My five star ratings are handed out quite sparingly and I do go back later and re-evaluate those in particular.