I could have been great if they had followed the book it was based on. Unfortunately they added in many scenes that were not in the book, removed ones that were and the entire ending and grand final battle have been made up (not in the book) and just does not work.
Add to that CGI that is not great, and a complete failure to convey the scale of the traction cities and what you end up with is a real let down.
I had expected better from Peter Jackson. It's worth renting, but not worth going to the theater for.