I read Dreamcatcher first a long time ago. I also saw the movie. I don't remember the order of if I did that relatively close in time.
I recently saw the movie again. And I was surprised. In my mind, the movie was like Aliens II and the book Alien the 1st. Lots of military and more action movie than suspense.
So, I decided to read the book again. I was wrong. The movie closely follows the book. It's an adaptation but a good one in my opinion. There are many changes to make it more palatable to a large audience. And all the alien-psychic mind reading stuff is probably harder to render correctly.
People always criticize Stephen King for the end of his books. But this one is quite nice and really better than the movie. But then, les goûts et les couleurs…

To conclude, a nice read. Maybe a bit long before the end.