I've been running for years and years - like, decades. When I first took it up (mid 80s), there obviously were no cellphones you could take with you. I always liked my music though, so I'd use a super-tiny AM/FM radio that I'd clip to my waistband and run the earbud cord up under my shirt. Once phones became of a small enough size to take with me (now we're talking 90s), I would ALWAYS have my phone on me. Always - wife might call, or maybe some disaster would happen and I needed to be reached. Bottom line - I always had my phone with me, mainly as my music source and also for emergencies.
Fast forward to last week and my purchase of the 645M - I got the M version with no real intention to use the music function, but it was a nice-to-have, just in case. Well I started to see how I could get some sub-portion of my iTunes library on the watch. Today I went on two fairly short runs, to see what a non-phone run experience would be. Amazingly freeing, is what it seemed like. My Garmin path was this: (after 20+ years of Polar devices) - started with the original square VivoActive, then to a 230, then a VA3, and now the 645M. I realize now how great it feels to leave the phone behind. Obviously a phone is useful on longer runs, also if out trail running in the woods, alone. For me, any run under 5 miles, I can leave the phone and water carrier behind (a Camelbak wrist thing, that I'd put the phone into the external pocket.
Freedom!!