I recently saw Harry Potter and the Goblet of Fire, The Chrnoicles of Narnia, and King Kong. All are very good films. I read the first couple of Narnia books at least 20 years ago and remember almost nothing about them (I do have them on the shelf to reread at some point), so I can't comment about the faithfulness of the adaptation, which is something I usually don't worry too much about anyway. Films and books are two different mediums, and changes have to occur if the adaptation is going to have a prayer of succeeding.
I remember only vague details of the Potter books. I enjoy them immensely as I'm reading them, but the minutia and lots of the plot details fade almost at once when I'm done. I don't know why, but it makes it nice to watch the movies because I have only the foggiest recollection of what is supposed to happen.
I thought Goblet of Fire was wonderful. I know that tons of stuff had to be cut out, but the little details left in -- like Neville dancing with himself after having such a wonderful time at the ball, or Hermione sitting on the steps and taking off the dress shoes that are obviously hurting her feet -- worked to make it all grounded and real. And Mad Eye Mooney was just marvelous. The effects work was top notch, and Ralph Fiennes pretty much nailed Voldemort. Highly recommended, but not for younger kids. The ante is definitely raised in this one, and it has some truly terrifying moments.
My son and I really enjoyed Narnia. Tilda Swinton is glorious as the white witch, and all of the child actors are more than adequate for their roles. I had some problems with the kids wandering around in a frigid forest in clothing that would have gotten them frozen to death inside of an hour or two, and what was up with Santa Claus showing up and handing out weapons (!)? But it was an enjoyable film that took the material seriously.
And finally, King Kong. If you love movies, you need to see this in a theater. Yes, it's long. Yes, some of the action is over-the-top ridiculous. Yes, some of the compositing work, especially during the brontosaurus stampede, is almost shockingly bad in a few shots. But you know what? The movie as a whole works. And Kong himself is the most incredibly realized special effect ever put to film. He's worth the price of admission alone. The recreation of 1930s New York City is a marvel, especially considering it was all back-lot work shot in New Zealand. The final scenes atop the Empire State Building are so realistic that I got some severe vertigo in a few instances. It's dizzying and scary and breathtaking. Don't miss it.