I recently added google's Play Music App (the one with the head-phones icon) to Galaxy 4. Curiously the music files I buy and download with it are not visible to any other music player, even the standard music player that is part of android JB.
This is a pain because in the car I use Car Music which is speech/gesture controlled so I can keep my eyes on the road.
To get around it, I used Googles music manager to download all my music onto my notebook and then copied onto the micro-SD.
I have also noticed that despite being set to only stream/download via WiFi, this app occassionally uses large amount of data. Not sure what is going on there. I have since turned off the caching function so will see if that makes any difference.
If anyone is contemplating downlaoding this app my advice is: DON'T


