The software is pretty good actually. Have a Galaxy Active2, two day battery life, good health sensors, and I find the interface incredible easy to use (after one or two day of adaptation). The fact that it is round is a plus for me too (it looks like a watch if you turn Always on screen, though I use it off in mine).

I don't know about the SDK though, but it does seem to have some apps (not that I use many of them).