Yes CarolC, Alan Rickman *is* worth going to the film for. Unfortunately, his delicious character is something we don't get enough of in the film.
I agree that this film won't hold the interests of many adults who haven't read the books. The plot is tortuous to follow for the non-initiated. As Graham has pointed out so well, there are many holes in the story that just make no sense whatsoever if you haven't read the book.
The film looks mahvellous, but the story line really doesn't hold up.
I too am all for anything that gets kids reading. Once they are accustomed to reading, it is easy to get them to read "good books" (haughty sniff).
And I can't second the suggestion of Susan Cooper's books enough. Dark, but delicious. It was my daughter's favorite series of "chapter books" we ever read aloud. She is a very good writer. So how about somebody makes a movie of her stuff?
Also, once the kids are older, introduce them to the Marion Zimmer Bradley "Mists of Avalon" books. They suffer from the same bloat that Tolkien does (why don't editors do their jobs?), but it is still grandly entertaining light and fun reading.