I'm a bit of a strange case: I'm a runner in the north east US and its been very cold until recently. So I've had my watch on the outside of my thermal sleeve, where it obviously can't read my heart rate, but I'm mostly using it for stop watch + it can monitor how much total time I'm spending running vs walking. So not much data from me about sweating + heart rate.
You can store music on the watch, but the sync is so slow and the data cap is 2 GB that unless I want to listen to the same playlist on infinite repeat its not viable. You'd also need bluetooth headphones, which is a purchase I don't feel I actually need... generally when running I'd rather be alert of surroundings less I get hit by a truck while jamming to a favorite track.
My use of music controls on the watch has been this: phone is sitting around office or home and connects to sound bar, I can leave phone charging somewhere while reading / studying / drawing and if I want to change tracks or volume its a flick on the wrist. Otherwise while in the subway + listening via headphones on a crowded train, can change music while keeping a grip on subway poles.
Yes, a recent update to AW enabled a feature where you can copy music directly to the watch and pair up BT headphones so you can leave your phone at home. No idea if the Apple device can do this, but AW people have been doing it for a few months now.