This is one of the best of the HP movies (I thought only #3 was better), and as an adaptation of the book is faithful and manages to distill it down to movie size without losing the power and imagery of the book. It is suitably dark (the cinematography is supurb in its bleakness) , the music is subdued but well done, and the screenplay and the acting brings it to the level of intensity the books had.
Are there things missing? Yes, without spoiling anything, I was a bit sorry that there isn't more about dumbledore's background that you see in the book, especially the tabloid attack on him (though that may be in part II, I have no way of knowing).
The movie is really about the three main leads, and they have grown into the roles literally and figuratively, and there is not a missed beat among them. There is nothing childish about their performances, and they have you believing this is real, not an easy thing with myself. And it is 2 hours 30 minutes yet it doesn't feel that long, it kept me into it until the very end.
One warning, there isn't a lot of lightness or humor in this (which is like the book), anyone expecting any sign of the gee whiz kiddie kind of thing is going to be disappointed, and I would be very careful about taking kids to this one who are younger then 11 or 12 (the pg13 is spot on IMO), make sure they have a tolerance for heavier stuff as this is not disneyfied in the least, unlike the first couple of movies.