The short version: If you are a JRR Tolkein purist, you will enjoy this slightly less than the LOTR movie franchise. If you are looking for a fun fantasy movie, this is a pretty good one.
The long version: A lot of effort (and padding!) has been put into making The Hobbit run parallel to the LOTR franchise, and it was pretty clear in closely An Unexpected Journey paralleled The Fellowship of the Ring. Personally, I think this detracts from the work overall. Also, there were some parts of the movie that were clearly aimed at the younger audience of the original story (the riddle-contest that went on for minutes, and the troll finding Bilbo in a sneeze), but there was also some rather gruesome violence that really had no business in an adaptation of a childrens’ book. That being said, it is a fun, action-filled fantasy adventure for young adults and up. The cinematography is outstanding and the action sequences well-choreographed.