Dawnguard is Bethesda's 1st DLC expansion for Skyrim. Priced at $20, Dawnguard is much more expensive than the other DLC packs Bethesda has released in the past. So, is it worth the price? Not at all.
Dawnguard's questlines are almost carbon copies of each other; each group has almost exactly the same quests and even if you join the anti-vampire Dawnguard, you still get an opportunity to become the new "Vampire Lord" transformation later on with no repercussions asides from random encounters with hunters, which pretty much eliminates most of the difference between the groups. The questline also forces you to associate with a rather poorly written character called Serana, a vampire with daddy issues. No matter what you do and how much you hate vampires, Serana WILL tag along and WILL NOT leave you until the questline is concluded. Thus, you are forced to listen to Serana's numerous problems and complaints all the time. Not to mention that Serana will pretty much screw over your stealth character on multiple occasions by charging into fights, but that's more of a problem with Skyrim's AI in general.
The new Vampire Lord form is not as fun as you'd expect. You are forced into the clunky 3rd-person view while transformed and your abilities are only useful at low levels. You also have to grind a lot in order to get all the powers, which is just tedious to do. The new Werewolf powers are even more boring, as none of them are very useful and are all things you could just do with Dragon Shouts. The new Crossbow weapon functions almost exactly as a bow, so it's not very exciting to use.
There are some positives to the DLC. The environments are very beautiful and have some fun secrets in them. Some of the new loot have interesting powers and abilities. While they are weaker than regular weapons, it is fun to play with their effects. There are some interesting lore tidbits to find through out the new areas as well.
In conclusion, wait for a severe price drop in Dawnguard before even thinking of buying it. There's not much to do and your $20 could be better spent on a full game rather than this DLC. Hopefully Bethesda learns from this and tries harder with their next DLC expansion.