I want to see it again (and re-read the book) but my first impression was that it was great. The original book is slim and has the disarming simplicity of an old fable or fairy-tale, so obviously the movie needed to expand the storyline a bit. There are a few overly-obvious points, but mostly the added material fits well with the spooky tone of the book, and sets the movie up for some great animated set-pieces. For example, the opening scene was new and, I thought, really grabbed your attention.
The movie theater was full of young kids - and for the length of the film they were eerily quiet. No one had to leave the theater in tears that I saw, but the film does maintain the fairly intense creepiness of the story and its almost harsh take on leaving childhood behind and entering the adult world. Of course, kids these days are pretty tough. At any rate: definitely recommended.
ps- the 3D version we saw was very well-done, although I wouldn't say it was a necessary ingredient of the film.