I've played with and it is highly accurate and I like that it keeps the data "on the device" and doesn't send it to any 3rd party server. To query the data you actually need an iOS app running on the device. You can look at your data offline too when you're on a mountain with no cell service.

However this kills half their market because of people like me who are invested in the Android eco-system so much that having an iPhone is a non-starter.

The Galaxy watches from Samsung are equivalent though and I'd say about the same accuracy for things like step count, calories, distance.