I've only played a couple hours of the new DLC, about an hour on each side. and I have a couple thoughts.
SPOILERS FOR THE FIRST BIT OF THE VAMPIRE SIDE: What you get the Vampire Lord ability, you're told to use the powers given to you in order to level said abilities up. But the first place you're sent, is a tight cramped ruin. And since you're forced into a third-person perspective, it makes it way too hard to control and aim with your bad-ass vampire lord. And you're even too tall to fit through several parts of the dungeon, even in the walking form, I found myself having to switch back to Human form rather often.
Some of the other things I've noticed in the first little while of each side: The Crossbow is bad-fucking-ass. The Vampire Lord form and abilities are all really cool. But overall I'm not seeing anything that's worth the steep $20 price tag. But that's just my first impression. I also felt that they handled the obtainment of the quest better than in any of their other games. In Fallout just having it pup-up the moment you start it up feels a little disconnected from the game. And I found it just way too hard to actually find any DLC quests in Oblivion. In Dawnguard, you walk near any guard in any town, and they just say "Blah blah blah Vampire Huntin' done be goin' down up in them there mountains!".
I'll be giving more thoughts once I've finished each side, since obviously everyone cares what I have to say about something all actual professionals will be doing the same thing, only a hundred times better.