Dawnguard adds a main quest with two factions, two big castles, an incredible follower (I love Seranna with my heart and soul) crossbows (which aren't poorly integrated, mind you), more side quests, expands upon the lore of the Falmer, has some pretty great new places to explore (well, one place, really), and it has vampire and werewolf perk trees. It has more content than Knights of the Nine while priced around the same at launch.
I'm merely regurgitated what was already said before, but it is definitely worth $20. The only game-breaking thing are the Vampire attacks killing off some people you may find essential. This wouldn't be an issue if Beth was able to do that thing they said they'd do where a merchant's son takes over the business or something along those lines...

Oh, and the fact that you can pay for Armored Troll companions is pretty amazing.