Those who loved Sir Peter Jackson's adaptation of JRR Tolkein's Lord of the Rings trilogy will love this movie. Those who didn't, won't. It's as simple as that.
It became a family tradition that I would take my family to see the Lord of the Rings movies at Christmas as they came out. My Brother and Mother, who'd both read the books as children, and re-read them since, loved the movies. My father, who hadn't, didn't. He even opined that Gandalf was derivative of Dumbledor, whereas of course, the opposite is true. He thought the movies over-long, muddled and unsatisfying. In the final movie, I could hear him mutter "and... cut" several times, as the movie reached a natural ending to the story, whereas those who'd read the book knew there were several story-lines still to come.
There are those critics who will think the movie "plodding" and over-long. That's a complaint with Tolkein's utter disregard for narrative arc. Indeed, it's this lack of tidy endings, and profusion of sub-plot lines that make the mythology so compelling. It's more like reality than many gritty cop-dramas or action movies today. There may even be purists who may take issue with the additions to the book's tale, but as these are telling back-stories and tying the Hobbit deeper into the Lord of the Rings narrative, it didn't bother me. I'm not sure why they feel the need to monkey around with Tolkein's prose though: "In a hole in the ground, there lived a Hobbit. Not a nasty-dirty hole filled with the ends of worms and an oozy smell... This was a hobbit hole, and that means comfort." Became some nonsense about "all the comforts of home". WHY?
As it happens, Gandalf turns up roughly where he does in the book. His absences are explained more fully than they are in the book. The "necromancer" in Mirkwood is clearly explained for what he is: The witch-king of Angbar, re-animated by Sauron's spririt, which Gandalf suspects long before the other guardians of Middle-Earth. We meet Radagast the Brown and Saruman. The Dwarves are all given characters (and regional British accents). Thorin Oakenshield is a properly hard bastard. Bilbo is well imagined. Many of the other characters are familiar from the Lord of the Rings. There are more comic moments than in the previous movie, but this too is in keeping with the book.
The company of Dwarves isn't the hand-picked band of mighty warriors that the Fellowship of the Ring was, but ordinary (if short) blokes united by faith and loyalty. This is a thread which runs through all Tolkein's work: the idea that free people thrust into extraordinary situations will do remarkable things. Tolkein never claimed to have been influenced by his experiences on the Western Front in 1916, but it's clear he was. He asserted there to be no analogy to the second world war in his books.
Gandalf's greatest insight is that Hobbits - a sort of idealised rustic Englishman were a better bulwark against evil than the great princes and warriors of greater strength and fame, who're too easily corrupted by power. This is perhaps the reason the mythological cycle of which the Hobbit forms a part is so appealing to the Anglo-Saxon world: it speaks to a dimly remembered folk-memory of doughty farmers and nascent local democracy dating from the dark-ages. The idea that we're free, and they're not.
There are those who'll complain about the CGI or the 48 frame-per-second technology.Some think the pace plodding. I disagree. I could have easily sat through the entire story told at this pace, and I'm slightly miffed I have to wait a whole year for the next installment. I will struggle to not buy the DVD so I can watch it before I go and see the next installment: Through Mirkwood, or whatever it is going to be called, and wait for the Director's cut trilogy boxed set in 2015 or so.
If you're an unashamed owner of the extended, director's cut boxed set of the Lord of the Rings, then go and see The Hobbit: An unexpected Journey. If you thought the Lord of the Rings to be an overblown fairy-tale, don't bother. Ultimately, you know the world, you know the story that's going to be told. Sir Peter Jackson has created another masterpiece.