The Desolation of Smaug is a better movie overall that its predecessor, but perhaps only by a thin margin. Because the entire trilogy was, for the most part, filmed all at once and has subsequently been tweaked through re-shoots and editing, there's really only so much director Peter Jackson could do to respond to the valid criticisms of the first Hobbit film. His choices had largely already been made; the die had been cast. And so most of those questionable choices linger and repeat here in the second film... but not to the extent they did in the first film. Hence, the result is a slightly improved movie.
This movie is still languidly paced in many places due to the financially motivated decision to expand a simple novel into a three part movie. This time around, there's a lot of Tolkien fan service that simply isn't needed. An early subplot involving Beorn the shape-changer, though largely lifted from the book, feels like a needless diversion. It feels no more necessary to the story that the visit with Tom Bombadil was in The Fellowship of the Ring (which was mercifully omitted from that film). Gandalf's side quest to investigate the rise of the Necromancer (not from the book, but from other Tolkien writings actually set decades before the events of The Hobbit) is surprisingly dry... or perhaps not so surprisingly, given the unavoidable prequel problem: everyone knows exactly how that all turns out.
Not all the added material falls short, though. In fact, some of the wholly invented material is among the best stuff in the movie. Evangeline Lilly's controversial character of Tauriel is a welcome addition. She's a far more interesting and meaningful character than Arwen was in the original trilogy (with no apologies to the comparably dry Liv Tyler), and while her storyline is a bit simplistic in places, she also gets most of the effective emotional material in the film.
The action sequences of the movie are generally better than in An Unexpected Journey. There's nothing here quite as prolongedly stupid as the Goblin Kingdom or Stone Giant sequences of the first film. That said, the "same kinds of mistakes, just fewer of them" problem I mentioned earlier shows up more in the action than in any other element of the movie. There is still a lot of ridiculous, slapstick violence that doesn't feel authentic to Tolkien's overarching "war is hell" philosophy. And the characters still are mostly immune to any injury that would lend any credible sense of danger to the proceedings.
But virtually every scene built around simple conversation works very well. The character of Bard, introduced in this film, is a sympathetic one, and the story in which his family shame over his ancestor's failure to slay Smaug plays very well -- though I believe it's a script invention with no basis in the original book. Scenes that show the corruption of Bilbo (by the One Ring) and Thorin (by his quest for the Arkenstone) effectively do what the action scenes don't -- show that there are consequences to the events of the story.
And as with An Unexpected Journey, the best scenes of the movie involve Martin Freeman as Bilbo interacting with a CG character. Andy Serkis' motion-captured performance as Gollum was the highlight of the first film (and I expect will remain the highlight of the entire trilogy, once it's done). Benedict Cumberbatch's work here as Smaug is nearly as good. The great dragon is not only menacing, but seems wise, spiteful, and toying. (For example, my memory of the book is that Smaug never understands the nature of his invisible tormentor, Bilbo, but in the film he seems to know exactly what the One Ring is and how it is being used against him. A nice improvement, I think.) There are a few bad lines of dialogue here and there (including a "your world will burn" taunt so cliche that I believe it appeared word for word in the 47 Ronin trailer that ran before the movie), but overall Smaug is a fantastically realized character.
All told, The Desolation of Smaug is a decent film. And as I've said, it's an improvement over An Unexpected Journey. I would give it a B. Still, I do think that prior film did lower the expectations. If hypothetically this had been the first Middle-earth film we'd seen since The Return of the King, I think everyone would still be talking about how it really was a disappointment after the superbly executed original trilogy.