Not to be mean or anything, but you must of ran through every quest there was and didn't search anything. I started at around 9am this morning and stopped about 230pm and I still haven't finished it.
Absolutely, I feel that I've hardly even scratched the surface of what Zion has to offer. I rushed through the neccesary main quests only, took in the story it had to tell, and got all achievements.
Later today or tomorrow I will really get to see how many hours of entertainment are in this DLC.
EDIT: @The Tribal - There is one new radio station when the DLC loads, but it's just to point you towards the DLC. I don't think any new songs were added.
The main highlights would likely be the new area to explore and equipment available. The story isn't as interesting as Dead Money's was, in my opinion.