I think this is a dangerous mindset to have, assuming that just because someone's bought a product, that gives the manufacturer license to collect data on my health habits. I would be much more receptive to an opt-in study.
Yes... and no. Don't folks get insurance premium discounts for using one of these things? If you are poor and having trouble putting food on the table, it isn't much of an actual choice.
Sure, now that's there, it could be useful. But why can't we ever go about collecting such data the right way? Opt-in, and for the right purpose?
Sure, but that's application. Great things usually can do terrible things depending on application.
At some point we have to take a leap of faith and acknowledge that some things, like heart data ingestion, is an amazing advancement for humanity. Lets not try and stop it, lets try to move it in the right direction. Ie, not private, etc.
Judging from previous performance of using statistical analysis to study health, I don't see it solving anything major.
Health is mostly a combinations of genetics and healthy routines.
People already have a good idea of how they could improve their routines/diets--they choose not to. Healthy routines are hard. So unless the massive data sets are related to genetics, I don't see them contributing much.
Also good luck "anonymizing" genetic data!
Edit: Also, turn ALL of this into a single step, and make it easier to find: https://blog.fitbit.com/go-incognito/