The dawnguard expansion isn't really that essential. A few of the included features you can find in the form of mods (such as smithing arrows and dragonbone weapons).
The story is interesting but I feel it is a pathetic excuse for DLC. A lot of the time you are visiting places that already exist in the vanilla world with a few new added areas. In the end, regardless of the side you choose, the outcome is the same, both paths end with the same scenario. To save you some time and trouble, without revealing too much, go with the Dawnguard, you get better benefits.
Considering the time it took to develop this was basically the same time it took to develop Shivering Isles, I am very disappointed with the amount of content added. Hopefully the next DLC (hearthfire) won't be such a disappointment. I only got it because I finally had something I could use my GAME points on and got it for £8
.