In case you have avoided Play music because it lacked true offline music support, the most recent update to the app may interest you.
https://play.google.com/store/apps/details?id=com.google.android.music&hl=en
As per the changes -
v5.3.1316M
* SD card support for offline music on KitKat (Experimental support on some older devices)
* Added ability to shuffle all songs from an artist in All Access
* Made it easy to share links of songs, artists and albums to your favorite social networks
/FYI
https://play.google.com/store/apps/details?id=com.google.android.music&hl=en
As per the changes -
v5.3.1316M
* SD card support for offline music on KitKat (Experimental support on some older devices)
* Added ability to shuffle all songs from an artist in All Access
* Made it easy to share links of songs, artists and albums to your favorite social networks
/FYI