I finally got around to seeing the Hogfather. It was good, but not as good as the book. I suspect that the problem may be that it is hard to translate Pratchett to the screen. The Discworld definitely can be a dark place. But it is also a very funny one. The film treatment had the darkness down, but lacked the humor.
It was marked as PG so I thought I would watch it with my boys however we only got a few scenes in before I decided that it was likely to give them nightmares.
Often the humor in Discworld is not in what the characters see but how it is described and what they think in reaction to it, and there are also frequent side thoughts and even the odd footnote (even some of the footnotes have footnotes). All of this is hard to get into a medium that uses images rather then words to describe, and considers it bad form to make thoughts audible.