Personally having recently gotten Dawnguard and completed its main questline as a vampire lord I think that it was better done then the main questline. for the simple reason that there was an actual consequence that I could get a feel of that we were trying to stop.

And in all honestly Harkon provided more of a challenge to defeat as a final boss then Alduin. And bar Blackreach/Any Dwemer ruin dawnguard had far more interesting locations to go to then the main questline, the Forgotten Vale and Soul Cain may just be my new favorite place.

Well, this is usually the case with DLC to Bethesda's games. Shivering Isles was probably 10 times better than Oblivion. In Fallout 3, Operation Anchorage also had an end boss that was actually difficult, the writing for The Pitt had much better writing than the original game, and Point Lookout gave a fresh new environment to explore. In another Dawnguard example, Serana is a character with character, unlike all the other followers in the game.