I ask because I've recently read a few books that didn't work for me. There was nothing technically wrong with the books, but a few elements stood out in a bad way, and it kept me from enjoying the story. Specifically:
1) Character issues. I didn't care for the way the heroine treated the hero, and I couldn't, for the life of me, figure out why he stayed with her.
2) Pacing. This book started out fine, but the last 20% became a mad dash to the finish line, with the plot unfolding in increasingly ridiculous directions, almost as if the author realized they had 'x' number of scenes left to shove into 'y' number of words.
Even though I wasn't excited about these books, I stuck with it and finished them because I figured they would be good examples of what not to do. It's like eating peas--I hate the way they taste, but I know they're good for me (or so I've been told--I have my suspicions on that front). Sometimes I can learn more reading a book I don't enjoy than I do from reading one I like. Still, since I read for pleasure, I don't do this very often.
What about you? Do you struggle through to the bitter end so you can use the book as a primer on what not to do, or do you drop a book that's not working for you because life is too short to read books you don't enjoy?