My favorite horror films are the ones that don't throw blood in your face. They don't torture their victims or cut their limbs off. I saw Poltergeist when I was 10, and it's still one of the scariest movies I've ever seen, because most of the real scares took place off screen, where my imagination could make up fates worse than what the filmmakers could come up with. Insidious, while not as good as Poltergeist, is still an effective scarer in that mold.
It's not surprise since it comes in part from the guy who created Paranormal Activity. But from his collaborators, the two guys that created the Saw series, it is. Even more surprising is that they are just as good with the psychological stuff as they are with the gore. Insidious is, at its core, a haunted house movie. A couple moves into a new home with their three kids, only to find they've got unwanted house guests. Soon, one of their sons falls into a coma science can't explain, strange things start happening around the house, and a group of occult experts come in to solve the mystery.
The less said about this movie the better. Just know that you are going to be scared, not grossed out. And honestly, I prefer it that way. With this film and Paranormal Activity before it, we're seeing a return of the scary movie that freaks you out with the unseen, that makes you yell, "Don't go in there." As someone who loves the old school, I couldn't be happier.