Anywho...praises to that quest, and praise to Dawnguard!
And yet some people start threads questioning whether Dawnguard is worth the money, some people want everything for nothing-truly amazing.
Lost to the Ages was the first quest I did in DG. After completing it, I couldn't believe it was considered "just a sidequest". That was a huge dungeon, with a great story and interesting characters (IMO). It was really fun and rewarding.

There were three more things in DG that made the DLC more than worth the money in my opinion. (These could be considered major spoilers, so I'll put them in spoiler tags so those who haven't finished DG don't get spoiled) :
Spoiler Jiub!!!
Spoiler When Jiub started speaking, my jaw dropped. His voice made my whole year. Thank you Bethesda!
Spoiler Meeting unchanged Falmer. Not ghosts, or going back in time to speak with them via Elder Scroll. Real, live Snow Elves. That made me so happy.