The Pebble Time 2 looks pretty tempting to develop a custom running/fitness app for. It's got heartrate detector, accelerometer, vibrator for silent feedback, plus an SDK that works with Linux.
I almost decided to make my own app recently (and was also considering the Pebble) to try and replicate the old basis Peak functionality, but when I found out that the Garmin health data was relatively open and accessible, l decided that it was easier to just make a new badge/gamification system for the Garmin data than to make an entire tracking app.
I almost decided to make my own app recently (and was also considering the Pebble) to try and replicate the old basis Peak functionality, but when I found out that the Garmin health data was relatively open and accessible, l decided that it was easier to just make a new badge/gamification system for the Garmin data than to make an entire tracking app.