I believe that people generally fall into two camps, those who will finish a book they have started no matter how terrible it is and those will give up after a half-hearted try.
A friend was talking about how she was trying to read a book series that some people she knew liked, and she got several books in even though it was terrible. I just can't do that. If a book doesn't grab my attention or is poorly written, I just toss it aside and pick up the next one.
Which one are you? Also, what is the worst book that you have actually read cover-to-cover?