I saw the movie last weekend. An extremely effective albeit overhyped horror movie, it stays with you far after the movie is over. My friend that I saw it with has been paranoid all week hearing noises and apparently seeing odd lights, and the night we saw the movie apparently his garage was robbed, which either happened whilst we were away or while we were at his house, except there was no sign of forced entry.
All in all, the experience and feeling you have after seeing the movie is probably more effective than the movie itself, but that is merely a backhanded compliment since clearly in order to be affected so the film had to do
something, and
Paranormal Activity will definitely stay with you far after the movie has ended. To me, that's the mark of a damn fine horror flick.