The dragon, ultimately, is the true reason to go watch this. More than the other films, this one deviates from the original book more than all others. But it deviates in a good way for the most part. As usual the plot and characters are all blended together in a great way. CG, music and acting are all great.
Downsides to this film include increased violence (Mostly against scary monsters).
Creative decapitations seemed to have been the theme, which is a bit unfortunate. As with the previous film they brought Orlando bloom back as Legolas. They just couldn't pull off making him look as young as he was in the original trilogy. Despite this he played his role well.
Also, depending on how you look at it, the ending could be good or bad. Lets just say that whereas the other films let you down gently, this one has a very abrupt ending.
All in all the film is an adventurous ride and quite entertaining. The characters continue to develop and change, as well as the overarching story of the rise of Sauron, leading to the Lord of the Rings.
I give this a strong PG-13 for strong combat violence and some disturbing imagery. Older teens should be OK with this.