When I first got my Apple Watch I was entranced. It is wonderful and I will use it as part of my MO forever. Saying that, I am a fitness fanatic and have gone through Fitbits and Loops and hoped that the Apple Watch would be a reliable HRM with just it's light on my wrist. I saw that the calories it said I expended were far less than what my chest strap said I expended. I know that I might be biased, but this was So Much. The Apple Watch was far more stingy about saying how many calories I burned at any exercise. I decided to try out the Apple Watch for a class, and then try it out for that same class the next week. The teachers were the same (Zumba and Yoga), the final numbers were very different. I don't have the precise figures but the Apple Watch was about 1/3 less than what I got on the Polar Bluetooth Heart Rate Monitor and Digifit. I was using the Apple Watch in its exercise mode and tried to choose an activity which would closely track the activity as possible (thought there is nothing like Yoga on the Watch). Digifit gave me far more exercise points than Apple Watch did. I think this is because the Polar Chest Strap is made to monitor your heart rate at all times when you have it on. Apple Watch only checks your heartrate at times during your time with the activity tracker on. So, if you have the activity tracker on, you might get checked on once every 15 minutes with the assumption your heart rate remains the same inbetween. It does not have activities for things like yoga. The Polar HRM does not care, it's going to give you your figures no matter what. So though I would love to use the Apple Watch for everything, I am going to track my exercise with the Polar Chest Strap.