The Hobbit is the only one of Tolkien's Middle-earth stories that I've actually been able to read in its entirety, primarily because of its simplistic style as compared to The Lord of the Rings.
However, that simplicity is also a double-edged sword that, in other circumstances, would prevent it from being adapted in any medium other than animation.
Regardless of how you feel about the decision to make a trilogy of 3-hour long movies out of a fairly short story, Peter Jackson and Co. have succeeded with flying colors in making the story adaptable in live action by augmenting its fairly simplistic narrative with additional material - original or otherwise - a decision that allows the core of the novel's story - which IS very much present in both this movie and An Unexpected Journey - to work in a way that doesn't feel silly, which, incidentally, is also what Andrew Adamson was able to do with his adaptations of The Lion, The Witch, and The Wardrobe and Prince Caspian (particularly the latter).
Two specific instances from this movie that demonstrate what I'm talking about are the introduction of Beorn and the Mirkwood spiders sequence where the spiders talk to each other. Both of these things could've made the film seem silly, but by not dwelling too much on Beorn and having Bilbo only be able to perceive the spider's chatter by putting on the Ring, Jackson and Co. were able to maintain the 'historical reality' tone they'd already established for the franchise with their LotR adaptations and their adaptation of the first third of The Hobbit as realized in An Unexpected Journey.
The Smaug sequences did seem to drag a bit in places, but were still very well-handled, and Cumberbatch was the perfect choice to voice the character, giving him a sense of menace and malice that perfectly fits with the tone of the film and makes you believe that he's a real creature that could've existed in our prehistoric past.
I'm giving the movie a 10 out of 10, although I did miss part of the ending due to some drowsiness and therefore need to see it again.