While, as a film critic, it is dangerous to make broad generalizations (all Vin Diesel movies aren't worthy of my time, all Fellini movies are egotistically maddening, etc.) I feel relatively safe in claiming that films adapted from Stephen King works fall under three categories. Category 1: Very good (Carrie, Cujo, Shawshank Redemption). Category 2: Generally sub-par (Thinner, Rose Red, The Langoliers). Category 3: Generally good except for the endings (this category is by far the most prevalent: It, The Stand, Tommyknockers).
Of course, there are some Stephen King adaptations that fall outside of these, but generally, in my opinion (which is the only opinion that matters, as this is my review), most fall within these.
The newest Stephen King adaptation is Dreamcatcher. I would have to say that it falls most closely into Category 3. However, while It was generally entertaining and scary until the monster turned out to be a giant spider very close to the end, Dreamcatcher is only really good for a half hour, and then inexplicably bad for the last hour and a half or so. I looked at my watch far more often than a horror fan should in a Stephen King adaptation.
Director Lawrence Kasdan (also producer and co-writer) is our guide into a world of four childhood friends from New England (hmmmm... can you tell it's King-esque?) who, through an altruistic act many years ago, gain the ability to read minds. On an annual camping trip to Maine, they are subjected to a snowstorm and then lots of paranormal stuff begins happening.
Morgan Freeman plays some government agent who works with aliens and UFOs, and the movie pretty much goes from genuinely scary and unnerving to weird once the sinister occurences are attributed to alien forces. Perhaps the reason Dreamcatcher is so bad is that the aliens, who are reeking havoc through a virus which gives its victims deadly gas, are revealed too early in the film. I don't like knowing what's going on. I liked to be scared by what I don't know.
Although I have not read Dreamcatcher, I've read enough Stephen King to know that the way he writes his "monsters" is, in literature, very creepy. However, when his film adaptations create them, they look too "literal". There are some weirdly funny and incredibly bizarre sequences in Dreamcatcher, and Jason Lee is funny as "Beave", but the movie was very bad. Just... weird and bad.