A series that immediately comes to mind is the Lord of the Rings trilogy. I found the books virtually unreadable due to the very poor pacing, but the movies were great.
Other than that, Children of Men. The book is a competent, atmospheric look at a dying world, but the ending was too abrupt and contrived, plus it didn't make any sense. The movie is very different from the book, but I felt it worked a lot better from a story point-of-view.
I also liked Order of the Phoenix the movie a lot more than the book. The book had major issues with pacing and believability in the character department (everybody is suddenly humping like jackrabbits, wth?). The movie excised plenty of the extraneous bits, which made the good bits stand out. Plus I was always a little confused by the climactic scene in the Department of Mysteries (at least I think that's what it's called), but watching the movie made me 'get' the scene.