I really loved the movie. I read the Hobbit when I was very little, I can berely remember a time when I didn't know middle-earth. And then, sitting there in the cinema, watching my childhood heroes come alive, was amazing. The beginning, with Bag End and the Shrie theme felt so much like coming home, I actually cried.
I loved that they kept so much dialogue from the book, especially the opening sentences "In a hole in the ground, there lived a hobbit...".
Quite apart from that, the film is beautifully made, Martin Freeman is perfect as Bilbo, out of his depths but still trying to keep up with what's going on.
It didn't feel slow to me, but I suppose it might be different if you don't know the books. I like that they took the time to set up the story, to show what's at stake. The meeting in Bilbo's house is long, and not exactly action-heavy, but it's not dull, what with that brilliant "Blunt the knives" song and the dished being thrown about. And then it goes serious again, with the dwarves singing in the dark, and its obvious how important this all is to Thorin. Yes, it's probably half an hour of just talking, but they talk about interesting and important stuff.
I love that hte dwarves have slightly more individual personalities than they did in the books, and how Gandalf keeps being a bit baffled by their antics. He's a good mix between friendly, grandfatherly mentor and powerful wizard involved in very serious business who should not be messed with.
The action scenes are maybe a bit over the top, but still fun to watch. They were of course expanded from the book, since Tolkiens descriptions of battles are usually rather short, he'd probably seen enough of war to know that there is little glory in it, but an epic adventure movie nowadays needs a few good, big action set pieces.
Another major change from the book is Bilbo's evolution. In the book, he was much more timid and only turned into a more heroic character once the group reaches Mirkwood. It's likely a consequence of having three movies, there had to be some character developement for him in this movie or people would have complained about him being a useless burden to the much more competent dwarves. The bit at the end, where Thorin hugs him, is sweet, and the theme of strong, battle-forged friendships between men is close enough to Tolkien's writing that it fits well.
Overall, the Hobbit is an excellent movie that shines in the quiet, dramatic moments without neglecting fun, epic, if slightly unrealistic, battles.