At first I had no plan whatsoever to buy Dawnguard because I wasn't into the vampires/ vampire hunters kind of thing, but I was a sucker anyways, and decided to buy the DLC when it first came out.
So far I haven't played on the side of the vampire hunters (Dawnguard) so I don't know what their storyline consist of.
Pros: Increases imagination and storyline with your character. More to do. Love the vampire lord transformation, especially since you can deactivate it anytime you want.
Cons: Broken radiant quest given by Vingalmo (One of the vampires in Volkihar Keep). The Vampire questline seems to be rather dull IMO. Lack of followers on the vampire side while on the other hand, according to uesp, 5 followers of the Dawnguard to choose from, and this also includes creatures. A lot of traveling, and less quests.
According to people here, Some experience unplayable bugs once Dawnguard is installed while others have no problems at all.
I give this DLC the rating of 3 out of 5. Although I do give this DLC a low rating, I still do like it regards. The price of Dawnguard is somewhat expensive in my taste, especially with such short storyline. I would say that this game is worth probably 10 bucks, not 20. Yeah I'm cheap.