Like anything depends on your personal use cases, or lack of them.
I have lots of 'little' use cases which combined make a smart watch a device to seriously consider, assuming it ticks the right boxes, which most of the announced ones seem to. As long as it gives a days good use use, having it charge overnight is not a big deal for me.
Several little use cases:
* Auto unlocking based on smart watch / phone proximity. I mainly have a PIN on my phone to provide some protection should I leave the phone somewhere, but that gets annoying if it's on my desk at home or work, when I'm there too.
* Heart rate monitoring for my treadmill. My old Polar chest band has given up the ghost, so I need a new one. The old one sync'd directly with my treadmill, and a smart watch based one (assuming its reasonably accurate) will be more flexible.
* Directions when cycling / walking without having to hold a phone or get a bike mount. Particularly in winter when it gets to minus double figures regularly.
* I'm hoping the electronic ticketing app I use for public transport will (eventually) push tickets to the smart watch, meaning less arsing around in winter to show a ticket every time I get on a bus.
* Real time public transport updates without having to arse around getting my phone out, especially in winter.
* Being able to 'screen' messages quickly to determine if I need to respond before arsing around getting my phone out, particularly in winter.
* Being able to find out the time (since I don't currently wear a watch) without arsing around getting my phone out, particularly in winter.
(For the record, during winter here the temperature regularly drops into minus double figures, making gloves mandatory, which makes smartphone interactions 'fun')
There might be no one 'killer' app, but why does there have to be? For me, solving a number of small issues that bug me almost daily makes a smart watch which solves them attractive.