I'm curious how good the heart rate tracking is. Most current fitness gadgets suck terribly at tracking heart rate while exercising. afaik polar or garmin plus a chest band is the only thing that does a good job, whereas most fitbit type gadgets are only good at calculating resting heart rates. If apple can crack that, they'll find a lot of purchasers. And garmin / polar will be roadkill: their devices are expensive, single use, a big hassle (that damn chest strap is annoying, plus now you have two batteries to worry about), with shitty software.