Even though I won't get one from the start because I usually don't do that with Apple products, I'm very interested in this. I basically need to see some real-world uses for this that could justify a $350-$400 expense since I would likely buy the larger of the Sport models.
I own a Fitbit, but I haven't used it for about a year. I dealt with some of the rashes and a splotch on my wrist that wouldn't go away for months had me freaked out. But I wasn't an owner of the recalled models, so I'm SOL. Fitbit refusing to tie into the Health app also bugged me. So for now I've been using what's possible with an iPhone 6 for step tracking, sleep tracking, etc. I had a nice heartbeat app that became useless once I put a thicker wallet case on my phone.
But outside of these uses, I'm not sure what it would do for me. Yelling at me to get up might be useful, but I actually have to get up a couple of times an hour, albeit for only a few seconds. Sometimes when riding a bus controlling music from my watch would be INSANELY easier, but that's a minor thing. Apple Pay also may be easier, but I'd settle for just more places to use it and my credit union joining.
Controlling my Apple TV and unlocking my car door are a couple of things that could land me. So could placing calls from this device like we can from iPads and Macs. I had a fun game of "where's my iPhone" last night, so it's nice to not need your phone for calls.
I find great use out of my iPhone, iPad, MacBook Air and Apple TV. My iMac served its purpose but right now is sick. I didn't buy a first gen of any of those except an iPhone when it was discounted to $250.