I think it would be hard to argue that it DOESN'T make you more fit? If you normally have say 500 steps in a day, and the FitBit convinces you (through gamification, visibility into the metrics, or whatever it may be) to now take 2,000 steps in a day, then it's not a "feeling like they are more fit", it will indeed make them more fit (assuming nothing else were to change). The health benefit in 2,000 steps vs 500 steps is probably pretty cut and dry.