I like Dragonborn, but I was very disappointed by Dawnguard. It's hard to go into why without spoilers, but I found the story to be ... not bad, exactly, but extremely forced. I also hate the constant vampire attacks on townsfolk. Too often, too many, and they can be surprisingly dangerous spawns, even at very low levels.
I do like a lot of the non-story content, though. There are some very nice new locations, and the expansion of options for vampire and werewolves is good. So is the ability to craft arrows.
All in all, I don't feel horribly ripped off for buying it, but I can't honestly recommend it.