Watched Sinister last night. It was decent. Good acting, and the first half was pretty scary. However, like most modern scary movies, it started off strong, and ended weak. Once the children started showing up, it became much less scary, and even kind of lame. The actual ending was pretty twisted in a good way, though.
Of course, I can't speak for everyone, but I personally think movies are much scarier when the "evil entities" don't get too much screen time. It's better when their presence is implied, more than outright shown. Insidious is another example that has the exact same problem. Starts off strong. Gets stupid once the paranormal investigators show up, and the evil things get much more screen time. It totally cheapens the fear factor.
I think it's what you can't fully see or understand that makes and keeps a movie creepy, IMO. Like the first time you watch Blair Witch or Paranormal Activity (except for the very, very end when Katie comes at the camera - LAME).
Hell, even the Woman in Black does it right. I think that movie is creepy as fuck all the way 'til the end, and it's honestly because the "Woman" doesn't get too much full-on screen time. I wish more modern scary movies would be like that.