The Fitbit Charge HR is continuing to surprise me; not only does the Heart Rate monitor work really well; but the sleep tracking knows automatically when I’m asleep or awake (unlike the Flex which required me to enable it).
Now I find out that the display is a real time display of data when used while running (on a treadmill, etc.) — you can actually see the numbers updating with each step — just make sure you set your screen to stay on.