Is DNFing a Book Giving Up—or Simply Respecting Your Time?

Every reader eventually encounters a book they just can't get into. Maybe the writing isn't working for you, the story is moving too slowly, or the characters aren't keeping your attention. The question is whether you should push through because you started it or simply put it down and move on. Do you think DNFing a book is giving up too easily, or is it simply knowing when a book isn't worth your time?