Version 2.0.31213 of Xbox Music for iOS allows users to cache playlists to their device and listen to them even if they currently lack a proper internet connection, for example when traveling by either a subway or a plane. Unfortunately, it's currently unknown when the Android version of Xbox Music will receive this feature.
Using the Xbox Music service is completely free of charge for the first six months. After that time passes, if the user doesn't opt for a paid subscription, streaming will be limited to no more or less than 10 hours per month.
Download: Xbox Music (iOS | Android | Windows Phone)