My FR 645M was Garmin's first watch (I think) to support music and there were a lot of issues with maintaining a decent connection to various earbuds (the forum used to keep a list of what headphones worked or didn't). I know that Garmin said that if you wore the watch on your left wrist -- which most people do -- and the receiver for your headphones/earbuds was on your right side -- which most of them are -- then there might be issues with maintaining a good connection. It was also dependent on positioning of your wrist relative to your earbuds. In my case, they worked well when I was running and my hands were held up in front of my chest, but I'd get breakups in the audio if I were walking with my hands down at my sides.
My question here is if this has been improved significantly on these newer watches. Reading the user manual the other day, I noticed the same warning in there about left wrist - right-side receiver, which makes it sound like things aren't that much better. Or are they?