This review has sold me on the expansion. Having a companion with some actual character (and maybe a growth arc) sounds like a revelation for a Bethesda game, too bad there aren't enough to make Skyrim even worthy of licking New Vegas' boots in the literature quality department.Dawnguard even manages to improve on one of its parent game's few weak areas by including a follower character with a history and some genuine personality. The follower characters from the main game are so typically unaffected by events around them, and so repetitious in their use of stock phrases that it's very easy to start thinking of them as being little more than walking luggage racks.
The tighter scope of the expansion pack, however, has allowed Bethesda to create a companion that feels a lot more like a person, and offers considerably more context-sensitive dialogue options and actions, as well as conversation that actually extends beyond, “I am sworn to carry your burdens”. The explanations, descriptions and backstory she provides to the player while out in the field happily make this character an integral part of the over-arching story in a way that isn't always present in the main game.
These guys will probably fix a lot in time.