I think all the gizmos (vivofit, fitbit, Mio Alpha, etc) can range anywhere from very useful to less than useless. It kinda depends on what you are using it for and what your particular motivation is. Some people are heavily and internally motivated others want them as an accountablity deivice. Neither is necessarily wrong, its just important to be honest with yourself about what you expect out of it and yourself.
Run like the wind, but never forget gravity.