Personally, I liked the beginning. It was fine and worked OK. The troll scene didn't. Three long movies from a short book, and they missed out on really developing Bilbo's growth. It's one of the book's key elements. Bilbo's is a placid hobbit who is fat and eats 10 times a day, and grows to one who by trial and error, guts and fate, comes into his own. The scene didn't really convey how out of place Bilbo is at the start. It shows he's a little awkward and thrust into things, yet failed by giving Bilbo Gandolf's role thinking up a cleaver escape. The whole movie missed out on developing characters well, with a few exceptions (not including Gandalf's thing with Galadriel).
My least favorite scene is during the mine chase when the wooden bridge fell 30 stories and they ride it down. Lost a lot of interest. It went from magical, heroic characters and suspenseful danger down to completely not serious. I expected it based on some of Jackson's interviews, even still it came up short.