As a rule, I trust people. This means that I take recommendations from friends and family and I will stick with a book to the end almost every single time. The added bonus is that if the book sucks I will lambaste whoever recommended the book and then feel right as rain immediately after
The only time I DNF was a self-published novel from a first-time writer. I actively pursued him afterward and gave my constructive two cents in the most polite way possible, and offered any help/advice I could provide. The passion was there, I said, but the editing was not.
Unfortunately, he replied in a less than pleasing fashion, and it took everything in my soul to not rate his book 1 star. Had I not been an author I'm sure I would have.
Oh, and now that I'm really thinking about it, I DNF a Sherrilyn Kenyon book. I bought three of her books on sale never having read her, got through half of one of them and promptly returned the two I hadn't cracked open.