It’s been good. I can’t really give you numbers because I typically throw it on the charger when I’m showering or sitting on the couch watching some tv. This allows me to wear it all night so I can use the alarm and see my HR values overnight. It takes the biggest hit when I’m running and listening to music. That’s to be expected though.
I think that's one the biggest advantages right now with the Series 3 Apple Watch, is able to stream music and running without your iPhone.