I am not one of those who think King is a great writer, his books are fine distractions for a few hours, and often have interesting premises, however they, in my opinion, fall apart pretty rapidly if you give them any serious thought.
This film suffers from being a pretty exact re-telling of King's story. If you have read the book, there are zero surprises, no changes, no altering of scenes or characters. This is a film made by King fans for King fans.
For the rest of us, there is nothing terrible about the film, it is a 'gypsy curse' horror, with its twist being the curse being something many people wish for, to become thinner. The final third has some severe structural problems, and the slightly forced ending seems to break the rules of this genre a little.
There are worse ways to spend 90 minutes of your life, but you might as well read the book, because there is nothing new here...