Better still, remember you'll be able to wirelessly sync you iPhone-stored music to the device, whilst you are sitting in the park.FALSE : it's impossible to wirelessly stream your music from your device outside the range of YOUR PRIVATE WIFI, if it's only based on AirPlay.
But you can now setup your iPhone as a personal (closed) wifi hotspot and then stream to your device using this? Your devices would have to connect to your iPhone hotspot, but it can be done.