Say you're reading a book that runs to about 400 pages. The first quarter is excellent, but over the next eighty pages it becomes repetitive, the plot lags, and you start to wonder why you should care. With just over half the book to go, do you a) put the book aside and instead read that brisk, funny Nicholson Baker novel you've got sitting on your book shelf; b) continue reading for another fifty-odd pages in the hope that things will pick up and with full knowledge that even if they don't you'll probably end up finishing the thing out of a misplaced sense of duty; or c) finish reading it because you've come this far and you might as well see how it turns out.
The book is The French Lieutenant's Woman, by the way. Perhaps I ought to add a fourth option, d) flick through until you find the sex scene, read it, then put the book aside and read the Baker.