In my experience the MMI works quite well with an iPod connected to the AMI as music storage space / jukebox. If you want Bluetooth streaming I've tried different AUX cables and AMI dongles and none of them work well. Lot's of glitches, lost connections and over all terrible audio quality. In the end I was so fed up that I forked the 200 euro's for the Kufatec solution that plugs into the MOST network. It integrates nicely with the MMI and offers BT streaming and an additional AUX connection, without the need to plug into the AMI port so my iPod still works.
The A2DP Plug and Play Bluetooth Music Receiver for Audi MMI 3G offers the possibility to play music from a mobile phone via the Audi MMI