Sony Music Unlimited has offered a new (catch-up) feature with the addition of offline access for music downloaded through the Sony Music Unlimited service. It's comparable to other streaming music sites like Spotify, MOG, Rdio, Rhapsody and Slacker Radio. Taking in all of the relevant reasons for offline access: travelling on a plane or by car in remote areas where there is no online access (or they charge you $$), underground locations like the subway (and Dick Cheney's underground bunker). Eliot Van Buskirk at Evolver.FM cites another reason, probably the most important one currently: "... cellphone providers are capping the amount of data you can stream each month." He also said that it cuts down on battery use, since you're not actively streaming to play the music, you're just playing music that's already on the device like an Android phone or Android tablet or or any of their own devices like the Sony Walkman or PlayStation.
The Sony Music Unlimited streaming music service will soon be available as an iPhone app and iPad app for iOS. MTV said that it's also coming to the Vita.
Check out more about streaming music sites in the Spacelab Streaming Music Guide.