I thought the quest was very atmospheric and I really loved the moths as well as the reading of the Elder Scrolls. I realize that the vampire attack was predictable but was also necessary, challenging, and fun. I also enjoyed talking to Serana at the end of the quest.
What's interesting about Skyrim is how utterly addictive the game is. I was ready to put it down a week ago because I'm having issues with the framerate and that stupid Vilkas/Farkas bug. I had told myself, "Just one more night then move on" even though I've yet to complete Dawnguard and I've been working on it for weeks.
But I can't do it. I'm on my fifth character, with about 500 total hours invested, and yet I'm planning my next character already!
Anyway, Bethesda, thank you for Dawnguard and for Unseen Visions. You guys are the best!