In the "olden days" I always finished books I started, no matter what. But the last 10 years I've been more critical in wasting my time with bad or boring books. For that I can thank Danielle Steel. I bougt "The Ghost" (10 years ago), thought (by the back cover) that it would be an interesting ghost story. Then it turned out to be some romantic crap about an elderly man wanting back his cheating wife. (Although it was clear through the book that a cheating husband is always a pig, whereas a cheating wife is always ok).
Well, I threw the book into the walls a couple of times, but thinking of the NOK 149,- (US$ ~30) I'd spent, I finished all the 432 pages. Whereafter I *burned* the book. After that, I am *very* reluctant to "impulse buy" books by female authors, authoring "bestselling ghost stories".
J.K Rowling being the exception
Anyways, as others here, I may stop reading and then pick ut the book after a few months, finishing them.