When it comes to scariness, Insidious starts out strong. I threw my hands over my eyes so often that I could hardly see through my glasses for the smudges. But about halfway through the movie, things start to fall apart and the whole thing starts to look like it was filmed in your local Halloween haunted house. Creepy wax figures, an over-used smoke machine, and olde timey childlike music are the main sources of fear after the half way point. Throw in a couple ghost busters who look almost exactly like Mormon missionaries (though too disheveled to be actual missionaries), and it's just not that scary after awhile.
Plus, who's the bad guy? The demon who looks exactly like Darth Maul (I am in no way nerdy enough to make up that reference on my own - someone else pointed it out and truly, the resemblance is shocking)? The old lady (who's picture is slightly too creepy to include here)? The wax figures? I don't know. I totally lost track of who I was supposed to be afraid of.
My advice: rent it if you want a few good scares. Just keep your expectations at medium.
Who to see it with: someone you want to cuddle with
Scariness level out of ten: 5 (mostly relying on shock value)
Rent it or see it in the theater? Rent it!
Creepiness factor when you wake up in the middle of the night thinking about it? 7 out of 10