I used to want to stick with books because I used to buy them from shops where they were so expensive! Now there's Amazon though, I only spend a couple of quid, if that, so I don't feel too bad. I don't have a lot of time to read books I'm not enjoying, so I don't see the point, I don't think they'll give me anything.
Still, The Book Thief is my favourite book of all time, and I initially disliked the beginning, so it kinda depends. You have to give it some time before giving up completely. I also love The Three Musketeers, but the beginning of that was difficult to get through, and it's a very drawn out story so you have to be prepared for the long haul. I'm also a great fan of the Falco book series (20 books in total), and once I got to the fourth book I wanted to stop because it was getting very same-y, but I carried on and the rest of the books didn't disappoint! So it really just depends on if you think the book is worth carrying on. If it makes no difference either way, or the plot doesn't particularly jump out to me, then I usually don't mind giving it to a charity shop or something.