I just finished Dawnguard's main story line for the vampire hunters, and I plan to do the vampire side on my next character. I've gotta say it was really quite good. The vampire transformation, the new perk tree for werewolves, and the crossbow are all really cool. I think the crossbow might be a little overpowered though maybe. I also love that they explained a bit about the snow elves because I was really interested in them as soon as I found out that they weren't just monsters. If you haven't gotten it yet, you probably should.
I've also been messing around with Hearthfire and its not bad. It would be neat if instead of set positions for things you could choose where everything went. Also the fact that you can't rearrange the side hall things is annoying. Other than that, its pretty neat as well, though probably only people who roleplay a little would really want it.