I wouldnt term this a horror movie in the true sense. The scariest parts were the scenes leading up to what was in the mist.
But this movie isnt entirely about monsters.Rather its about humanity, how people react in crisis, religious fanaticism, desperation and hopelessness. You will come to strongly hate and love the characters.
The only flaw I found and what I thought was crucial was the explanation. What exactly is the mist? Where did it come from? What about the monsters? The director and writer tried to explain this in the movie, very weakly and poorly and their explanation came off as ludicrous and unbelievable.
Nonetheless this is a film you wont forget so you should watch it if you can.