I looked at the health app today and it reported that my average bpm is 59, with a range of 32-179. That range seems a little sketchy to me. So, I grabbed Garmin chest strap and put that on and did a side by side. It was pretty close. The Garmin reports 1-10bps off from the apple watch, but usually the apple watch can get within 1-2bps after a moment of syncing, and almost every time it was lower than the Garmin. I trust the Garmin because I've seen them compared to an EKG, and both kept reporting the same information. Now the next test would be to take it out for a run and see how the Apple Watch does.