I agree with KRF. The only other differences are the ways in which some characters die and the omission of other characters' deaths. Most of the omitted deaths happen at the end and don't affect the outcome. To me, it's always an advantage to read a book before seeing the movie adaptation.
Also, I agree with others that this is a faithful adaptation, all things considered. It's interesting to note that when the movie came out, the book had not yet been published.
I'm not sure if this is true, it seems like Carpenter had filmed it before Christine was published but released it months later.