The Health App has been a win. Tracking meals and what have you is straightforward.
But the real win is the gamification of the Global Challenge. While completely meaningless in any real-world sense, somehow keeping the step score and all of the silly achievements and missions help to quantify and motivate.
Recently, via the headphones, I've taken in some outstanding podcasts while out walking. (No specifics, lest I violate the HN orthodoxy.)
Overall, any app that gets us up and moving is a positive thing.