It depends on whether you are a quest fan or an add-on fan.
The Dawnguard questline is not worth the money, imo. It only took me 5-6 hours to complete for both sides. While there are a couple of cool, new dungeons to explore, there just isn't enough to the story. Not to mention the DLC is full of quest-breaking glitches. Like Skyrim had before any of the patches.
But it may be worth it just for the add-ons. It gives you new outfits, new weapons, new creatures to fight, arrow smithing, new shouts, new spells, the ability to change your appearance, etc.
Overall, I'd say it was barely good enough. Despite the "meh" questline and glitches (which seem to mostly affect only the dawnguard quests), the add-ons make-up for it.