I can't actually review this film as I normally would. Your level of enjoyment resting heavily on your knowing nothing about it.
Going in cold is the only way to really see it. However, I will make fun of the characters. Because hey, it's the Cavalcade...and one of them is an ass.
Structurally, this movie sharply constructed around building tension. So much so, in fact, that in a different movie, all the cat scares would just be a waste of time.
A "cat scare" is the shock of sudden movement or a noise that jolts the audience, breaking the tension before ramping it back up again. Usually, its a false scare in the form of a startled cat jumping out from under the bed that causes the hero/victim to jump, relax, and then promptly get stabbed in the face by the Killer in the Closet™. However, In Paranormal Activity, the cat scares don't break the tension-instead turning it up even more.
Scary things have been happening to the main character Katie for her whole life, and she has kept it from her boyfriend until they moved in together, and where most men would break up with her, this guy tries his hand at becoming the next Felini. Because he's an idiot.
This is all in the first ten minutes, so we're still fairly spoiler free.
The female lead finds her keys on the ground, and she didn't leave them there. Not scary. Later a door slams on it's own. These incidents build and build like this in ways I won't spoil until neither we, nor the main characters, can handle the tension. The girl gets anxious about the paranormal presence, the guy makes a joke that's not funny. As the audience gets more on edge, he makes some obscene gestures with his hands. He tries to get her to have sex on camera, SCARY THING. Wash, rinse, repeat.
Later on, he more or less beats his chest and says, "No demon messes with my girl!", as though wearing his white baseball cap wrong and popping his collar is going to stop something that's been around since the 1500's or so and is obviously bitter about it.
A claustrophobic little film, it takes place in roughly 4 rooms of a single house; and as the tension continues to build you'll start looking for things to be scared of, scrutinizing the screen and thinking, "Ohmigod! Was the cabinet open before? Did the lights just flicker? Holysh*tbirdeatingcrappants!"
Coupled with amazingly visceral effects, great atmosphere, editing intentionally slapdash, and performances that are so "blah" in their presentation that these characters can only be real people-Paranormal Activity is a great shocker. As an added bonus, when you see it again, it's hysterically funny. Calvacade this one with The Blair Witch Project or Quarantine for more psudo-reality film scares.