I also liked the movie. Shyamalan is a master of setting a mood and keeping you guessing. Even if the major, obligatory twist at the end was not much of a surprise to some, he still successfully keeps you guessing during the small moments (why does that person have that expression on their face, what is around the corner, what is that person about to touch, is this about to be a scary moment or a false alarm, etc.).
As far as the major twist, my two friends guessed it very early into the movie. I actually considered the possibility even before I saw the movie, and yet oddly put that possibility out of my head while I was watching it. That's a compliment to the performances, the script, and how well the mood and setting are directed that I can guess the twist correctly before yet dismiss it during. When the twist was revealed, I obviously wasn't surprised anymore, but I was still satisfied. It was a good twist to a good story.
I would say that the twist is better than the one in Signs, that the whole ending of the movie is better than in Unbreakable, and the overall story of the movie is almost as good, if not on par, with Sixth Sense. The performances are very good, so the movie stands well on its own.
I only have three minor quibbles about the movie. For a village that has no electricity, the lighting occasionally looks Hollywoodishly artificial. There's a blind character in the movie who seems to get around almost too well without vision. And, it's hard to tell the exact scope and professional diversity of the village's population, and sometimes I found myself wondering if they really had all the material and human resources necessary to be completely self-sufficient. If they don't trade, shouldn't they have had crops all over the place, or at least some gardens here and there? I don't remember seeing any.
I would say it's an A- movie the first time around, but I'll say it's a B+, because it's one of those movies that once you've seen it one time, it may not hold up as well on repeated viewings (with the tension gone and the secret revealed). I would say it's the fourth best movie of the summer, behind Spider-Man 2, Fahrenheit 9/11, and The Terminal.