The fact that they could fit a 1,000 page book into three movies, and they made the same amount of movies for a 300 page book, says it all really.
For me, the decision to split the second movie into two parts is when it all went to hell.
There is absolutely no climax in the second movie. They finally get to the Lonely Mountain, they meet Smaug, and suddenly.... a weird/random chase scene ensues that serves seemingly no purpose, and when Smaug finally flies out to attack Laketown (what I REALLY wanted to see), the movie ends? WTF?
It's one thing to end on a cliffhanger, but this literally ended in the "middle" of a scene; it was absolutely dumb. Not to mention the preceding action setpiece was so weird and dumb and completely unnecessary. (And fun fact, completely made up and completed like two weeks before wrapping production on the movie, when they realized they were making a third movie.)
And where does that leave us? With another 3 hour movie with basically no story left to tell except a battle scene. And the Laketown attack comes and goes in about 10 minutes' time and is completely forgotten by the time the battle starts. It was absolutely MORONIC to move that scene to the third movie. The equivalent would be to have The Two Towers end the moment they blew up the Deeping Wall at Helm's Deep.
Anyway, that's my two cents. Rant over.