A few weeks Bob wrote a post about surprise endings:

I’ve thought about this particular Sixth-Sense example quite a bit. I think part of what made it such a great movie was that the evidence was all right there for us to see the whole time. Compare that to The Village, which–though not a bad film–was not the Sixth Sense. At the end of that movie, I did not blame myself for not having known the ending. Yes, things were linked, but I didn’t kick myself for not having known.

That’s not entirely it for Bob, either, as he also believes that it works if it “forces a reevaluation of a relatively large number of facts.”
I’ll start by saying that I still haven’t seen The Sixth Sense and cannot comment on its relative value (though I do know about the surprise ending).

I agree with Bob and would add that the surprise ending ought not only to have clues along the way, but ought to explain something that you either didn’t understand along the way or chalked up to innate weirdness.

There were two surprises in Memento and a lot of resolved mysteries. The answering of the mystery of how Teddy ended up in Leonard’s crosshairs wasn’t so much a surprise I guess but rather a mystery solved. The entire movie was a succession of mysteries getting solved (“Why am I here?” “why is [something] like it is?”). The first was how precisely the black and white scenes merged with the color scenes and that it was all part of the same timeline (I had thought that the black and whites were just flashbacks to establish a backdrop).

The bigger surprise, though, was Leonard’s culpability in his wife’s death. That didn’t do as much for me as other surprise endings have, though, because it didn’t answer any questions that I was asking. It did shed some light on past events, but in some ways it raised more logistical questions than it answered.

Fight Club was slightly better in that regard. There were a lot of weird things going on throughout the movie that the surprise ending answered. Eventually the viewer just resigned himself or herself to the idea that the movie is just weird and surreal and let it go because it was entertaining and interesting. The revelation about Tyler Durden, though, made sense of the oddities of Marla’s behavior (and in retrospect made her a more sympathetic character) and of other seeming loose ends. It did raise other questions, though (did the people around him really not get wind that he was crazy?).

Another example would be Jacob’s Ladder. Leonard Maltin complained that it “negated the entire film” but in my mind it explained more than it negated. It explained all of the oddities going on and either answered the mysteries or established that there was no answer to them. Wild Things is another one with a surprise ending. The ending didn’t explain that much about the preceding events, but it was interesting to have everything you watched adjusted to a different narrative and adequately explained (as opposed to Memento).