Believe it when I see it
But it would be interesting to see what Apple can do to make the concept more appealing - is the core tech not useful or is it all about the implementation?
A device which is waterproof and can continuously measure heartrate, as well as recording sleep and activity levels and including a pedometer, AND DOES ALL OF THIS SLICKLY, could be a hit... I don't want to have to tell it "I'm going to sleep now" or "I'm swimming now". Slickness and polish are Apple's strengths and for something you wear on your wrist all day, tiny design nuances can make or break the experience.
But I don't think chaining it to an iPhone within 10 feet is a successful strategy, it MUST have standalone functionality. But then Apple just mentioned that whole "devices knowing what each other are doing" thing last week which could be ideal to let your watch know when to switch to autonomous mode and when to silently sync with your phone - leave your phone in your gym locker and the watch takes over, come back from the pool and by the time you pick up your phone, all the information has synced across. That sounds quite Apple-y to me.