Agreed. It seemed like King was in a hurry to get it over with. There was such a long gap between books 3 & 4 and another between 4 & 5. I don't think he ever wanted to finish the series, but felt he was obligated to do so.
I am not sure how they will handle book four, Wizard & Glass, since it predates the others. But on that book alone, King could have written a sequel. There is so much information that is missing between Roland's youth and his quest.
While that's probably my favorite of the seven, it could probably be covered as a series of flashbacks over the course of the three films.