There's something extremely magic about knowing who is calling you when you're cycling and therefore whether to stop or not to take the call.
Also, changing music or volume when on a bus without having to do that thing where you lift your bum to get your phone out of your pocket is, well, better than it sounds.
I have been wearing a Fitbit for over a year, my wife for 6 months. I can't imagine life without it now. The Big advantage is it keeps me honest. I aim to always do 10,000 steps a day, 100,000 steps a week, and I can see if I am short at any time. It is easy to convince yourself you have done more walking than you really have.