While by far not the best film I've ever seen-it's certainly not the worst. And it is one of the best Stephen King adaptations in some time. Although (unlike most films) the time frame of the movie is stretched out... And a slightly more in-depth plot is established- it remains very close to the original story.
It has some of the regular bumps and jumps that you expect from a 'horror' film- and as a result- becomes incredible dull and predictable during those times... But the movie surprisingly shows it's flare during the times that it becomes more psychological- relying less on cheap startles and truly becomes an unsettling tale of what it takes to bring a man to the brink.
If your looking for a movie that will have you guessing at every turn, or for a non-stop thrill ride- don't bother with this one.
However, if you like psychological/horror thrillers (Identity, Suspect Zero, Seven, Ju-on) which built on story, and take their time to do so... Give it a watch- it's worth the time.