I can honestly say that I've finished every book I've started, even if I hated it in the end.
Usually though, after I suffer through the first 100 pages, I catch the rhythm/cadence and vocabulary the author (i.e. Thomas Ruggles Pynchon, Jr.) is using and start to enjoy what I'm reading, but not always. I'll suffer through it because I'm frugal or stubborn.
I can also honestly say that I would not read any books that I had no interest in, or appeared beyond my scope of understanding in the first place.
** Correction **
There was one book that I didn't finish. My sister visited me about 10 or more years ago and had a Rocket eBook reader. She let me use it for a few days and I got half way through a book before she left. Can't even remember what book it was. I liked it, but probably was too cheap to go out and buy a book that I had already read half of.