Brandon, if this film is still playing anywhere near you, get your ass out and see it. If not, pick it up first chance you get. It's probably one of the best horror/ghost movies I've seen. Period. I think it ranks up there with The Others and The Orphanage in terms of inventiveness, but is still its own thing. And it's scary.
What can I say about it that won't give too much away? It starts out as your standard haunted house movie, things moving around, strange voices in the baby monitor, shadows moving across the windows, etc. and the pacing is probably better than average. Like I said, the slow tension build reminded me of The Others, so it's very effective. BUT, at a crucial point in the plot, the wife says, "I can't spend another night in this house." And the husband says, "Okay, we're gone tomorrow." What? One of the key draws of horror films is the audience's sense of superiority over the characters: they often don't do the smart thing and get the hell out. That convention serves the audience in a few ways, but an important one is that it lets you detach from horror to some degree because, well, you would never make such a dumb decision in the same circumstances.
But in Insidious, everyone believes in ghosts. And while you're watching it, you will, too.
So the family moves to another house, but, you see, there's still a good hour or so left in the film, so you know there's a bit more coming. And I will be very surprised if you anticipate what's coming.
There are cheesy moments, but they're actually nice and even a little nostalgic because the rest of it is so good. And I could have done without the ending. But the important part in a horror film is whether or not it's a good ride, and this one most definitely is.