Bought one of these from ebay - cost £140, including shipping. I now have it installed in place of the 6CD changer.
Really impressed with it so far, having all my music in the car is awesome, sound quality (to my ears) is as good as the CD player was and the menu's are easy to understand, you get an American voice saying the name of each album/artist/playlist as you run through the options.
The install was really easy, pretty much a direct swap for the CD, you do need a special cable for the Audi (they use a round connector as opposed to the oblong one that the VW's use). Mine came with this cable but I am going to swap it for an OEM Audi one as the quality of the cable is a little lacking.
In all highly recommended but would never pay the £500 + that Audi want for the unit.
Cheers,
Greg