In high school I had a teacher who would say that it does not matter if we know the ending of a story if the story is well-written. The enjoyment is in the story as a whole. Any story that is 'ruined' simply by knowing the ending obviously wasn't much of a story.
twooffour's argument doesn't hold water. The students in the study read a dozen different stories by a number of quality authors. And 11 of the stories were rated more enjoyable after knowing the twist. To say that the cleverness of the twist makes a difference misses the point. The students were rating the enjoyability of the books overall.
I think the cleverness of a twist might only be determinant when that's all the story has going for it, like the punchline of a joke. We may find that the more clever the twist, an individual might rate a story differently than a story with a less clever twist. For instance, "A Chess Problem" featured a greater gap in enjoyment than "Rhyme Never Pays". Maybe this has to do with the cleverness of the twist. But in the end, both stories were rated more enjoyable after knowing.
I, like RedFive, enjoy trying to figure out the mystery and being surprised. Maybe I would nonetheless rate a story/film more highly if I knew the ending. I think a number of the reasons in the article make sense.
Would Star Wars be more enjoyable if someone told the audience ahead of time that Leia was Luke's sister and Darth Vader was their father (the way many experience nowadays)? It is a qualitatively different experience from just seeing the evil guy in black stroll down the corridor and strangle a man. It doesn't necessarily make Vader less bad, but changes perceptions of many events.
Still at the end, I don't think people rate Star Wars more poorly for knowing these things. Maybe people would rate it more highly simply because even though they know the twists, they are more engaged to see exactly how it's sorted out. They are anticipating Vader's turn to good and taking enjoyment in knowing Leia is Luke's sister before they do (haha, you kissed your sister!). It's a different kind of enjoyment - and one I'd prefer on second viewing - but not "worse."
I suspect many people would say they would prefer not to know the twist ahead of time even though they enjoy the stories more after knowing. I'm totally comfortable having this cognitive dissonance.