Actually I use my MP3 player with ear phones most of the time on my 2012 FLTRX. I wear a full face helmet and at highway speeds the sound quality with the system on the bike isn't bad, but isn't great either.
I never even had a MP3 player with me on previous rides although now I do find it nice to listen to some "tunes". At times I find the system on the bike distracting especially if I'm relatively fast on a "twisty" and concentrating on the road and not really hearing the music, more so than the ear plugs.
There has been more than one occasion where a song with heavy bass popped up and I could "feel" it in the bike and would roll off thinking WTF just fell off.