I finished Dawnguard a few nights ago. It was pretty good. I'd say I got my moneys' worth. But not as good as the F:NV DLC. Their two large new areas had only a fraction of the things you could do in F:NV's DLCs.
Since I've never been into Vampires I chose the Dawnguard side (I love the crossbow). I might go back and do it as a Vampire just for the hell of it when I'm bored.
I liked that Serana was more interesting and had a backstory and things to say than any other followers you got with vanilla Skyrim. Again, more like F:NV's companions (I just wish I could tell her to wait and she would listen to me). It seems where Skyrim excelled over its predecessors was when it used ideas from F:NV. If only they'd made regenerating health a Restoration perk.
I'm waiting for there to be some meaty quest mods instead of the endless armor, house, weapons, etc. mods we're still getting.