Our church went, and I had many of my younger children attend and most did well through it. I advised their parents weeks ahead of time to read the book together - children are not as scared of things when they know what is going to happen and have already formed the image in their mind. One family brought their 4 year old, something I would not have been comfortable with, BUT they read it together every night and the boy was very familiar with the story and all it's creatures. He loved the movie and was only afraid a one scene - in the beginning of the movie, there is a scene not in the book. He wasn't prepared for it and it scared him.
There is an intense battle scene, and the creatures are quite scary looking for a child that young. But if the child has read or been read the book they should be prepared for everything that happens.
(I should also mention that 3 of my parents saw the movie a few days prior to make sure their child could handle it, that's also a great idea)