Look, I'm buckling up for a massive backlash here, but I feel this is something that needs to be said.
Much like the game it is designed for, Dawnguard is alright. The story is pretty interesting, (even though it is totally making it up as it goes along), the High Vampire powers are pretty damn cool, and I appreciate the Werewolf overhaul to make it worth a damn. However, we have to bring up the elephant in the room.
The price.
Dawnguard is clearly doing it's absolute best in the marketing to look like a Shivering Isles-esque expansion, with loads of new places to explore, buckets of new loot, and everything else that made Shivering Isles so damn good. However, do not believe this. Dawnguard has about as much content in it as one of Fallout 3's downloadable content pieces.
The biggest dissapointment is that a huge amount of the added content of Dawnguard does not, in fact, take place in a new location, like Big MT or The Divide in Fallout New Vegas. No, most of Dawnguards content comes from dungeons inserted into the main map of Skyrim, most of them built from very similar assets to the base game.
Translation: A signifigant chunk of Skyrim's content is spent in the same cookie-cutter dungeons you were bored of months ago.
Before you angrily type in your rebuttal, there are new locations in Dawnguard. Fort Dawnguard and Castle Ican'trememberthename serve as the hub for the Human and Vampire questlines respectively, plus, you do get to go do different locations during the main quest. However, you don't get to stay there very long, only one is particularly interesting and different enough from the main Skyrim landscape to be worthy of a mention, and the game does nothing with them, basically using them as locations for fetch quests.
However, the thing that kills my enjoyment of Dawnguard is the choice. Much has been made of the player's ability to side with either the Vampires or the Vampire Hunters. However, this choice is very poorly handled. In a game with this choice done well, like the aforementioned Fallout: New Vegas, Vampire: The Masquerade Bloodlines, or, hell, even the Civil War choice in Skyrim proper, the player is given ample time and oppurtunnity to learn of the factions available to them. This cannot be said of Dawnguard. You are forced to make the choice knowing very little of the Dawnguard, and fuck all about the Vampires, so it is literally impossible to make an informed decision on your first run-through.
Couple that with the Crossbow being underwhelming as an addition (it's basically an inferior version of the bow), Dawnguard is a merely average piece of DLC, but for just how bloody expensive is, twice the price of any of Fallout:NV's infintely better DLC pieces, it's not worth the money. If it ever goes down in price, yeah, why not, it isn't bad per se, but right now, this is something only hardcore Skyrim fans should consider, and even then, they should probably think pretty hard about it.