As far as a review goes, my non-spoiler recommendation is this: did you watch The Hobbit: an Unexpected Journey? Did you enjoy it? If you answered yes to both of those questions, then go see the Desolation of Smaug. You were going to do it anyway, but now at least you know I think it's worth going see the next one. I'm happy to put your mind at ease.
If you didn't see the previous movie, go see it first and come back. These movies kind of rely on each other, so I don't recommend skipping around.
If you saw it and didn't like it, you probably won't enjoy this one very much either. I mean, I personally enjoyed this one more than the previous one, but not enough that I think someone who didn't like the first one is going to suddenly be wowed by it.
Anyway, that's my spoiler-free review. It you're concerned about spoilers (either for this movie or the likely events of the next one), now is the time to turn back.
***
I've heard the Peter Jackson adaptation of The Hobbit described as fan fiction. I agree 100%, though I should be clear about two things:
1) Any adaptation in which the original author is not directly involved is, technically, fan fiction. For instance: The V for Vendetta film is fan fiction, since Alan Moore was never involved in the adaptation. Same for The Watchmen. Also, almost any given Disney movie. HBO's Game of Thrones series, on the other hand, I would not consider fan fiction since George R. R. Martin is quite involved in the production.
2) There is nothing inherently wrong with fan fiction.
These Peter Jackson Hobbit films are really good fan fiction. Or, at least, they are what I look for (and aspire to) in fan fiction. Rather than making huge changes to the core story, this adaptation looked deeper into its source material and expanded on the content, giving us more of what we want to know without compromising the characters.
In the Desolation of Smaug, we spend a lot of time with Legolas, whose involvement in the events of The Hobbit was implied in The Lord of the Rings, but never mentioned explicitly. We also have a new character, Tauriel, whose relationship with Legolas will likely pave the way for Legolas's friendship with Gimli later. And then you have a few characters whose involvement with the story has changed dramatically, such as Bard and Kili.
All of these changes seem important to me, not just as a way to pad out the trilogy and turn what was a short book into three massive movies, but mostly to increase the viewer's investment in these characters. I remember reading The Hobbit and only really being invested in Bilbo's journey and, perhaps, in Gandalf's appearances. The dwarves, except perhaps Thorin, were basically faceless allies. Bard was just some guy. The elves and Smaug were just obstacles: threats too big for little Bilbo to handle, so they were dealt with by other people.
My favorite change was definitely Smaug. Any dragon can be scary, as I try to relay in my D&D campaigns. However, intelligent villains are one of my favorite things to watch, and I thoroughly enjoyed watching Smaug toy with Bilbo, completely confident in the fact that he holds all of the cards and nobody, not Bilbo, the dwarves, nor the people of Lake-Town had any chance of stopping him.
One final note: I've never finished the Silmarillion, but it was interesting to recognize the Arkenstone as one of the legendary Silmarils. At least, I think that's what it was implying.