I've always appreciated a good twist ending, as it can enhance later reflection on the film. A good example is in the "Usual Suspects" when SPOILERS! near the end, the cop says the Gabriel Byrne character was really Kaiser Permanente, and I was all "Oh my god, what an awesome twist, I can leave the theater now!" and as I got up to beat the crowd it then turns out that the Kevin Spacey character was in fact Kaiser Permanente, a double twist! My mind was officially blown, right there in the aisle, and other movie patrons had to step over my comatose form when the credits rolled. I was officially snookered by that film.
On the other hand, when "The Sixth Sense" came out, I remember seeing the commercials where the kid is saying to Die Hard that he "sees dead people, and they don't even know they're dead!" I thought, "hmm, does that mean that Die Hard is a ghost in this movie? That would be a twist, since he's married to Demi Moore who starred in the movie 'Ghost' with Dalton from Road House, what a clever meta twist!" But then my friends told me that "Sixth Sense" had a great twist that'd blow your mind, so I saw it and was a bit disappointed that the twist was one that I'd anticipated. Good film otherwise, though.
Now, how many films could have been improved with a great twist?
1) Batman--turns out, he's really Spiderman.
2) Platoon--they were really in Ohio the whole time. It was a hunting trip, and the Viet Congs were really Asian American park rangers.
3) Planet of the Apes--it was Earth all along, but only a small part of it. When Charlton Heston continues further down the beach, he finds out he's in 1960s Malibu and walks onto the set of a Frankie Avalon beach movie.
4) The Godfather--Vito Corleone turns out to be really senile, and his crime fambly is actually a dentistry school. The students all call him "Godfather" because it's the only way to get an A in class.
5) E.T.--the E.T. wasn't an extra terrestrial after all, but a very, very dirty kid with a speech problem.
6) Red Dawn--the whole thing really took place in Russia, and the guys we thought were the Russians were the Americans and vice versa.