I'm another one in two minds over Jackson directing this. On the plus side we know that this will give a consistent look to Middle Earth and I think that was one of the things he got right with the LoTR movies (except the wargs and one or two little things).
What I didn't like were the changes made to some of the characters like Faramir and Theoden to name but two. These went from being great captains and men of action to being extremely wishywashy. I hope something similar doesn't occur with Thorin. Other changes like those made in The Two Towers spoiled that movie for me. That warg attack scene was completely pointless. Also the Elves at Helm's Deep. They were only added in so that Liv Tyler would have a bigger role in the movie. She was then edited out in post production with some reshoots taking place after the release of Fellowship and the feedback the changes made to her character in that movie was received.
So with Jackson at the helm should we expect to see Arwen at the Battle of the Five Armies? Perhaps Bilbo will be the one to shoot Smaug. Will Gollum follow Bilbo all the way from the Misty Mountains? These are the type of things I dread and I have little faith in Jackson keeping faith with the source material.