Fallout 3 had great add ons. But New Vegas has been hit or miss. Dead Money was terrible, Honest Hearts was okay, and Old World Blues was actually really good.
Dead Money felt rushed, it was like they wanted to get something out right away so all they did was tack on a maze. Which essentially is what it was.
Honest Heart was good, but then all of a sudden it was over. Maybe I rushed, but it seemed like I was just given the missions and it was over.
Old World Blues was a lot of fun. And the fact that you can go back and forth after you have finished it just makes it better. Now I can go find all the things that I missed.