Just got back from seeing. Very fun; in many ways stronger, though in a few weaker than the first film.
First off, good god are the sections with Smaug fantastic. I knew it would be good, but within five minutes I was ready to declare him the best LOTR film villain. Voice work was tremendous as expected but the expressions of him were a really pleasant surprise. Definitely hyped to see what he does in the first act of There and Back Again. The interesting characterizations of all the various other players (The Master of Laketown, Bard, Thranduil) also really fleshed out Middle Earth too.
The more consistent tone was a nice change of pace from the constant shifts of An Unexpected Journey and the build towards an actual real conclusion (the Lonely Mountain) was clearly preferred to the more awkward finish of the first film.
It does suffer in comparison to AUJ with too many simultaneous plot threads and slightly less organic character development. The stuff with Tauriel and Kili almost works given the chemistry of the actors, but still comes across as rushed and given the major plots going on with Smaug and the Necromancer a bit out of place. I suspect much of it is leading towards the elves involvement with the battle of the five armies, but we'll see.
Overall though very solid, and with enough elements to put it above the first film.