Insidious has some legit scares, I'll give it that. But at the end of the day, when the movie goes down the route of "Astral Projection", you're going to lose quite a lot of people along that way.
Insidious is an interesting ghost story because it takes the idea that was bought to us in "Paranormal Activity" of a person being haunted, not a house, and it furthers it by having us actually encounter the other side, here it is called "The Further". I have to admire James "Saw" Wan, because he definitely stripped things down to an old-school horror level (the opening theme credits are some of the best I've seen in a recent horror film) and the first act of the film is enough to get me going. I enjoyed everything up until the explanation of "The Further" and "Astral Projection". Sure, I understand the concepts being laid out here, but that doesn't mean that I have to accept or agree with them, and I don't. The second half of the movie bogs itself down into a ridiculous and silly conclusion, which almost seems as if the script was written in a scatter shot manner of "let's see how I can quickly wrap up this to get it filmed".
Still, I can think of at least half a dozen moments that had me pop just a tad, and unlike some of his previous films (the awful "Deadly Silence" comes to mind), Insidious for the most part delivers, even if only it delivers half of what I wanted out of it. At the end of the day, Insidious offers just a hint more then your average fright fest, but doesn't have a decent enough pay off to be overly worthwhile.