I have watched most of the Christie murder mystery films several times each, and as long as I have a year or two between viewings I remain quite incapable of remembering whodunnit. Instead I happily succumb to Christie's red herrings and the visual whims of the director to lead me off the beaten path.
I've had the same experience with the books. After I read all Agatha Christie murder-mystery books, I started re-rereading them, and could not recall who dun-it.
I have to say I think it's not great literature if it's so easy to forget the plot.
