My experience is that it's terrible. I've found it's off by up to 50% (low) - i.e. I run 10km on treadmill and the watch states just over 5!
I've recently switched from Samsung Health to Gear Tracker II - https://play.google.com/store/apps/details?id=com.limegreenv.geartracker
When you complete an indoor run in this app, it automatically gives you a "calibrate" option, which allows you to adjust the distance for your activity and saves your treadmill adjustment (e.g. if you calibrate + 10%, it will use 110% of the logged distance for future runs both for data posted e.g. to Strava and for the on-screen displayed data while running).
Great little app.