I recently had a full heart work up at a place that had me hooked up to an EKG machine while on a treadmill. I had a Band on and compared the data over time and it was consistent the whole time within just a couple of bpm. I was extremely impressed by how accurate it was.

It was only one day, but I left pretty confident in the data.

It's a lot more accurate when you set it to track an exercise. I believe it decreases polling frequency normally to save battery.

I have some weird heart issues and it can have trouble with that as well. If it's not set to track an exercise, it's frequently reading 10-15 bpm lower than my actual heart rate.

Not OP, but can answer. At its best, the band tracks relatively well to my chest strap. However, it's not 100% consistent - Some days the band seems to bounce around while my chest monitor is steady in the expected range. With that said, the band is close enough that I roll with it alone for my normal short runs(~5k) and only bother with the strap for longer distances. YMMV.