All it does is emulate a CD changer so the HU can accept audio on the changer input. You don't get any integration, but then you don't get brilliant integration with any of the solutions out there as the HU's are so featureless.
It has the mp3 player built in, so you use USB sticks, don't think you can plug an additional MP3 player in, as then you'd have to drive the HU from the additional player, which is a right ballache.
It's basically a mini Phatbox with no hard drive and no voice prompts, good compromise IMO.