I have an RCD 510 and a 7P6 bluetooth handsfree kit in it and the BT streaming works well, although its not ideal as you cant play/pause/skip like I had hoped. the unit cost me £200, so not that cheap, and an hour or so fitting myself.
Cant say I have tried any other method, although a friend has a cable input on his Audi radio, but I'm not sure how similar it is to the Golf.
Also I guess another advantage to te BT method is you then have a handsfree, which is the main reason I did it.