I've found mine is accurate as long as it is snug. But if it's not snug, it can be way off.
Also, when I first tried it I had wrist pain, and learned that I actually was "wearing it wrong". Moving it further up my arm fixed it, and now it's literally my favourite device.
I'm talking about Series 2. Series 3 isn't available yet. But Series 3 is supposedly same size as Series 2, except the thing that sticks out of the back is "2 sheets of paper thicker", to use Apple's new contribution to the imperial system :)
So my guess is it will be the same: just as good as my Polar chest strap when snug, and off when, well, off.
Also, when I first tried it I had wrist pain, and learned that I actually was "wearing it wrong". Moving it further up my arm fixed it, and now it's literally my favourite device.