But, HealthKit has received its first big blow recently, because Fitbit has said that it won't be adding compatibility for HealthKit. Fitbit is the most popular activity tracker brand on the market, with a number of wearables, but in a forum post it was made clear that HealthKit isn't in the plans:
Of course, the case isn't exactly shut on this. The forum moderator said that the company is open to reconsidering, but it needs to be convinced first. So, the question was put out to the community, asking what they would want out of HealthKit integration, and why they see it as being worthwhile. If you're a Fitbit user and you want deeper iOS support, you're going to have to appeal to Fitbit on this one.
If you check the source, users have already flooded the comments. As of this writing there are 41 pages of comments, most in favor of HealthKit support, and over 600 votes for integration. The general response is that users want a unified solution and if HealthKit can be that solution, developers should jump on board. We'll have to see what Fitbit's response will be.