I am a big fan of the Elder Scrolls games and I like Skyrim a lot. I also have the Dawn Guard add on and it is challenging and fun to play. My only concern is that once the download is in place and you reach a level 10 with a character the download just takes over the game and it becomes all about vampires.
Very quickly, vampires will start to attack your towns and kill villagers off. This includes vendors from the stores and can greatly limit your ability to trade goods and buy items.
I suggest that the download might be best left until the rest of the game has been played out as it is going to demand attention.