These always struck me as a fad waiting to die, but I'm not trying to be the usual Slashdot curmudgeon, so I'll ask: what are the killer features of a smart watch?
I mean, there aren't any. And that's OK. There aren't any killer features to tablets either, but we have them too. They are marginally useful, and I have one. If I lost it I would probably replace it eventually, but it's not like I have a backup like I do my smartphone...
For what it's worth, it is marginally useful to me:
1. When the phone rings, it buzzes on my wrist, which is far more reliable to notice than a vibrate / ring happening in my pocket while walking. I'm more likely to notice / catch the call while it's still ringing, and I can see if it's anyone I care about, or just another recruiter before reaching for my pocket.
2. When I get a message, often it doesn't need a response, so I can see it on my wrist without again faffing with the phone, unlocking it, etc.
3. Citymapper has very good turn-by-turn navigation on the watch, that is again much easier to glance at to see where your next change is and how far, etc.
4. The face is always on, and therefore much better for things like countdown timers when doing something active like cooking.
It's more about many many very little conveniences, which make it worth putting on in the morning for me, and buying if I have some spare cash for a toy. There aren't any killer features, but that's alright, there's still a place for them, not everything has to be 'disruptive'.