I say "based" because, while they do follow the adventures of Bilbo Baggins, Peter Jackson and his writing team took a few liberties. When watching the third and final installment, one has to take into account that this was the final part of a novel. It was not meant to be a complete story into itself but the final climactic conclusion.
The negative reviews made several points that I think are unfounded. First off, I did not see five endings to this. Rather, I felt as if this movie took its time to wrap up all the loose ends nicely in one big climax. It wasn't quick, but it sure felt that way for all that happened in the course of the final battle. Second, while I feel the battle and death of Smaug would have been a great ending of the second film and then the recovery afterwards a better place to begin this film, I understand that it was a way to keep audiences coming back for the full conclusion of the story. (As a writer who has broken up a novel for people to sample more for free, I can see the method behind this.) Finally, the reviews I had read said that Bilbo was hardly in this movie. Um...he hardly has a part to play in the battle of the five armies and goes home in the book. I thought his increased role in the movie fit perfectly.
The writers put a lot of effort into bringing in some of the outside (beyond The Hobbit book) events that tie this one very well into the Lord of the Rings movies. The battle of Saruman, Galadriel, and Elrond against Sauron to save Gandolf was a way to show them as more than we've seen previously. They actually fought! That was a small pleasure for me. Badass Galadariel ;)
The emotional links were much stronger in this trilogy than in the Lord of the Rings, imho, because of the liberties the writers took. I could have done without the love triangle of the movie-only character of Tauriel between Kili and Legolas, but I saw the purpose behind it as a means to show more depth in Thranduil, who gets very little mention in the book and certainly not much depth. And we see the mental illness that the gold causes Thorin and how he finally overcomes it. The deaths in the end also have greater meaning because of the care taken in adding character depth.
The one thing I could have done without was Alfred, who turned into no more than comic relief.
All in all, I enjoyed the movie and I look forward to buying this on blu-ray to watch back-to-back with the other parts.
I must admit that I probably enjoyed this more than many because 1) I have read the book a couple of times, although not for several years now, and 2) I had watched the first two movies in the last week to refresh my mind so that it would feel more like I was watching the conclusion of the story than a whole separate story.
Take this for what you will, but in my opinion, it was a well-done conclusion to the story.
The trailer for this part:
The trailer for this part: