So I do like mechanical watches and have a few of them, some vintage, some recent, none really expensive. I find those small movements utterly charming.
I always wear a watch, for me a phone never replaced it.
Then I received a stainless AW as an xmas gift from my wife and I must say that after initial doubts i found it really useful and pleasant. I currently wear it often during working days and business trips, my mechanicals are mainly worn in weekends now.
Battery life is not really an issue for me, it works all day, and I charge when I sleep.
But the true killer app is only in its infancy: health. A heart beat sensor is the beginning. it is well known that apple hired several people specialising in various other kind of sensors. FDA approval permitting these could be a crucial feature for commercial success since everyone wants to be and stay healthy.
Btw, several sources have estimates of 11-12 million Apple watches sold last year. I would say that is not bad for a new product category. If this is a flop, I wouldn't mind failing with 11 million pieces of anything.