I always liked to finish any books that I started in the past, but then I read one a year or so ago that I really struggled with and even though I finished it eventually it really put me off reading for a while and I didn't pick up a book for a number of weeks afterwards. It took me a while to get my reading mojo back, so from then on I decided that if I really didn't like the book then I would just give up on it and move on to something else.
Even now though I do tend to try and finish books if I possibly can, but the 50 page rule is a good one so I may adopt it in the future.