It's very hard to follow if you have never read the book. It's ok for Harry Potter's reader but definitely they left out important parts from the book. It would be better if they made it a 3 hours movie or even split it into 2 like what they will do for the last book. Should watch it just to prepare for the last movie.
You will be disappointed if you just want to watch a thriller. This movie requires thinking so don't go if you just want to watch a scary movie/thriller. Frankly it's not one of the best from M.Night Shyamalan. I would say it's probably one of his average movies.