One of the things you have to accept about 'possession' movies is that they are generally similar. It is, therefore, what differs that usually determines whether they are decent or another forgettable story.
This movie actually has something that makes it different - the ending is actually not bad at all. The problem is though, almost the entire film until that point is so poor.
The dialogue come across as though written by someone who does not know how conversations should sound. The characters don't react to anything that happens - you have shocking revelations and events and they just continue on as though someone has told them the weather forecast.
The concept of this story was decent, they just really needed someone who could actually write dialogue to fill out the spaces between plot points. The dialogue made the acting - which wasn't fantastic - look and sound even worse.