I've been known to really dislike a book but finishing it anyway because I hate leaving book unfinished. HOWEVER, If I really dont like it, I end up skimming the book till the end to make sure that I'm not missing out on something amazing. Usually not the case but I choose to give a book the benefit of the doubt!
This is a tough one... I always *try* to finish a book, but sometimes I just can't, and sometimes it has nothing to do with the book itself, life just gets in the way and by the time I have time again a new book is out that I am interested in. Whenever I win a book or am given a book by an author or publisher though I finish it regardless because I know it was given in the spirit of writing a fair and honest review.