I love Garmin hardware as it is super reliable. Software is more meh.
Also with this we get a music player which obviously would be suboptimal to have a cable hanging off my wrist (though Bluetooth could be cool but a bigger battery hog). This could also be interesting to monitor running form at some point too.
But I wish it could play music from Spotify, specifically Spotify in offline mode... which this can! I'm sold.
I think the Core is a step in the right direction, depending on how extendible it is with 3rd party apps. I love that the GPS is in such a small device and that it doesn't seem to suck the battery too much (as advertised at least). Sucks it doesn't have LTE as music streaming over 3G is painful to say the least. 4GB isn't much space to store music or podcasts either. While I'd love to ditch my phone for runs this doesn't allow me to do that quite yet.
Yeah I know back in the day we would just run with our little Casio watch, but I'm spoiled now :)
So now it's a <$170 device that has optical HR and GPS. It may not have some of the Garmin features (ANT+ is a maybe, GLONASS isn't included), but it adds a headphone output, connectivity and I suspect it will ultimately have a stronger app ecosystem.
If they've implemented optical HR well, it could be a very compelling alternative to the less specialised Garmin devices.
Running for 3+ hours was iffy with any of those devices because they'd eat their battery. Want to take your phone on a kayaking trip? Well sorry, guess you need a drybox and an extended battery!
The sad thing is that I actually like to listen to podcasts while I run. And, even though Google added podcast support to Play Music, podcast cannot be sync'ed to the watch. So I still have to carry my phone.
You can send a SOS and listen to spotify.