I thought this movie was pretty well done. Effectively scary in parts, paired with some solid characters that you actually get to care about. If I had to knock it for anything, I thought it could have been a little bit longer, it would have been nice to spend a little more time with the family before all heck breaks out. The acting was solid, all the way around, although I thought Vera Farmiga was the real standout.
As for the true story thing, it didn't bother me a bit. For those that believe in this kind of thing, it adds a foundation to the story. For skeptics, it might be something to scoff at and write off. But, it's not a documentary, it's just a movie, meant to entertain. I don't recall people throwing the same barbs at Fargo, which was also supposed to be a true story.