From a post on the ResearchKit blog:
Based on preliminary data for the Asthma Health app, over half of our users not only complete the e-consent process, they also come back the very next day to use the app. This is a very high rate of return for any app, let alone a health-related app.
Of course, consenting to a study and participating in one are two different things. Asthma Health needed to be as "sticky" as other popular apps in order to generate the required amount of study data. Excitingly, results have shown that users are as engaged (or more!) with Asthma Health as they are with games and social networks. Our working theory is that Asthma Health users are motivated by the goal of supporting research that helps the entire patient community. We plan to test this theory more extensively in the near future.