I always read the last few pages before I purchase a book. I like to know where the author is going before I invest time or money. While I know that many times authors are anticipating the reader's surprise (and actually working hard to construct it), I don't think that many understand that the book they imagine never matches what springs into the reader's imagination when they read his/her book.
Marion Zimmer Bradley used to tell the writer's that she mentored, that the book the writer writes is not the book the reader reads.
I think it's kind of the same as riding roller-coasters, some people crave the excitement, the exhilaration, and other's don't at all because their emotions don't match those of the people that do love roller-coasters.
According to an estimation I saw, as many as 40% of readers admit to reading the ending first. That's a pretty hefty minority.