Now that the Dawnguard DLC for The Elder Scrolls V: Skyrim has been released on PC too, you might be interested in reading some additional critiques on it, like this piece from Atomic Gamer, which conveys an overall unimpressed tone and awards the digital expansion a meager 6 out of 10. Here's an excerpt:
Is Dawnguard worth your money? Well, yes and no. Yes because there's some really interesting lore content for Elder Scrolls fans; no because the developers start throwing around priceless artifacts for literally less than the cost of four walls and a roof. Yes because you can become a vampire and gain special abilities as one; no because being a vampire is only really about combat, as you can't do any of the really interesting vampirish things in, say, Vampire: The Masquerade Bloodlines. Yes because there's new loot, spells, and abilities; no because most of that new stuff isn't as good as what you likely already have.
Yes because it's more Skyrim, no because it's really only that: a bit more Skyrim. For twenty bucks, I was expecting more.