How accurate would you say the heart rate sensor is with the Apple Watch? I know there's a fitbit with an optical HR monitor as well but it is apparently very u

How accurate would you say the heart rate sensor is with the Apple Watch? I know there's a fitbit with an optical HR monitor as well but it is apparently very unreliable. That's one of the reasons I ended up going for the Garmin Vivoactive + heart rate strap - accuracy of results. Do you know of any comparisons between the Apple Watch sensor and a strap?