I'd heard a lot about Paranormal Activity, from 'The best/scariest movie since The Exorcist' to 'So scary it was making people throw up in the aisles'.
It was very creepy at parts, even scary. It was very good. There were parts that had me actually tensing up due to creepiness or suspense.
But they didn't turn on the lights. Throughout it all, when the man characters were scared by something in the middle of the night, they didn't turn on the lights. It seems completely unrealistic that someone as scared as they seem to be would stay in the dark. If most people are mildly freaked out, the lights coming on are the first sign. Granted, it's a popular horror movie trick to keep the mood, but it just felt unrealistic.
Also, the end was not quite what the movie seemed to need. After acquiring a rougher copy of the movie, it seems that when the movie was given the chance for the big screen, they made some changes. The ending that they show in the preview was cut, though it was by far better, and they originally turned on the lights.
Suffice to say that if you can get your hands on a copy, it's worth watching.