[RheingoldRiver]

I think good advice is something along the lines of: "Find a good book discussion group" (for me this is /r/Fantasy). "Then, if you aren't liking a book somewhat significantly into it, explain to people there why you don't like it and should you keep reading it."

This method is pretty effective in a good discussion group because if you have explained your reasons well, people will understand your reasons for dislike and know whether things change; the payoff is worth it; the author improves; etc.

One example is The Licanius Trilogy. A lot of people say, "I read book 1, I hate it, should I finish it" and depending on how much they hated book 1 and their reading speed people will give differing advice. For example if you can finish the trilogy in 5 days (this is about how fast I read it) I would say yes, the ending is fantastic but the books are pretty dreadful in terms of prose quality and character depth. However if this will take you 6 months, no, bail out now and spend your time better. Read a plot summary on Wikipedia.

Reading is very much a social activity and much better when treated as such imo.