It did make some changes to the book, primarily though not exclusively the ending. It also played up (ahem) a not so subtle Ho Yay element that wasn't really there in the original story if I recall correctly.

That said it's still a good movie. It stars Gandalf (!) (Ian McKellen) and he does a very good job. The kid is more disturbing in his evil in the movie than in the book.

Not the best adapatation of King's work but far from the worst.
FWIW, I think King liked it.