I have mentioned before my old friend Mad Richard (I recorded his most glorious moment for posterity here). The common theme throughout his adventures, from his refusal to stop drinking (following doctor's orders) because he believed he had two livers  to attempting to train an insect he thought might be a flea into jumping through hoops , the recurring theme is a total inability to link cause and effect, combined with the unwavering belief that he is essentially indestructible. Generally, it's pretty fun to watch, though this is somewhat less true if it takes place whilst you're in a car and he's driving.
I mention Mad Richard because he reminds me so totally of the hero in Paranormal Activity, which I dragged Tiger to this weekend (sorry for the nightmares, honey!). Spoilers follow.
First off, I enjoyed the movie immensely. It's basically a text book example of a horror film that has a series of increasingly freaky events take place in the same location and then runs through them. It's hard not to view this as somewhat derivative, it's true, but it does it so well that I have no trouble forgiving it. I was particularly impressed with the fact that it managed so well to walk the fine line between explaining so much the unsettling mystery is lost and explaining so little it becomes impossible to tie the story together. If one considers a film to be a jigsaw puzzle, it's important that the viewer has enough pieces to feel like they can make a sensible guess as to what shape the missing pieces take (compare this with, say, Ringu II, in which it feels as though someone has mixed two entirely different jigsaw puzzles together and then handed you 30% of the resulting pile). Paranormal Activity manages that very well.
Part of it's skill lies in using its conceit (a couple attempts to videotape the strange goings on in their house) to its advantage. The static nature of the camera set up to film their hallway from their bedroom (and useless a tactic though I'm sure it would be, I'm pretty sure that if I believed a demon had set up residence in my house I'd be sleeping with the door closed) allows the film to ratchet up the tension very effectively. The same location with escalating events, as I say. This is helped by the fact that the location in question includes the characters' bedroom. One of the few things I liked about Ju-On was the scene in which a woman was killed by a ghost that slid into her bed; making what is supposed to be the place we feel most safe suddenly dangerous. This concept of the invasion of privacy is one of the film's best aspects, and is one of the reasons I think the repeated comparisons to The Blair Witch Project are so lazy. You need more than just a horror film shot with camcorders, I think.
On the other hand, there is a definite connection between the two films (and many other horror films) in one important way: the narrative is entirely dependent on at least one character being unbelievably fucking stupid. In The Blair Witch Project, that person is Mike, a man so entirely unburdened by intellect his response to finding a map difficult to read is to throw the thing away. That one act is so breath-takingly idiotic that it threw me right out of the film the first time I watched it, and it took me a long time to get back in.
As idiotic as Mike clearly is, though, he's small potatoes compared to Mika, the vacant-cranium numbnuts "hero" of Paranormal Activity. This is a man whose response to a demon haunting his home is to mock its power. A man who is so convinced that the psychic who visits his house is a charlatan that he immediately wants to disobey the advice that is proffered, namely: don't use a Ouija board to try and natter with the demon. A man who, having promised his girlfriend that he won't buy such a device (Katie being something like 15000 times smarter than he is, and that's despite the fact that the demon is clearly targeting her) then goes out and borrows one, as though her only problem was the expense involved, and then immediately asks her to help him use it. A man who attempts to interrupt the argument that follows Katie's horrified realisation that the demon used the board to spell out a message whilst they were out by asking if she'd possibly help him translate what the demon had said. He won't leave. He won't call the experts. He won't listen to his girlfriend at any point throughout the entire film. He claims to be in total control, in the same way Mad Richard will claim "It'll be fine!", only whilst in the middle of a situation in which his girlfriend's life might very well be in danger.
It's a real shame, because it does real damage to an otherwise excellent film. It doesn't exactly ruin it - the rest of the film is too good for that - but it makes it much harder to buy in. And horror films need you to buy in, especially those that are trading in "realism". I've mentioned before that I think people make too much of characters behaving "foolishly" in horror films. People go to watch a film about a mad axe murderer, and judge each character's actions as though they should be aware they are in a horror film, forgetting that they themselves hear noises in the dead of night, and don't run out of the door screaming in case it turns out to be a serial killer. Mika, though; Mika is just dumb by any metric. Not leaving the house because a door is mysteriously wobbling? Fine. Not leaving because your girlfriend has suddenly developed the world's creepiest case of somnambulism? OK. Not leaving despite the fact that you have videotaped evidence that an invisible creature is leaving footprints in your bedroom while you sleep? Get. The. Fuck. Out.
Anyway. If you have a higher (or I guess even equal) tolerance than I do for characters who are too simply stupid to be believable, I really recommend giving the film ago. Be warned, though, it's not an easy task to go to bed once you seen it.
 And one kidney. He described the day he was disabused of this particular notion as being akin to the moment one is informed that there is no Father Christmas.
 This was done by filling his bath with water, placing the wee beastie on a sponge floating in the centre (to prevent escape), holding out a hoop of some kind, and then prodding the sponge until the terrified arthropod leaped desperately to safety. Experimental conclusion: subject drowned.