Mercedes has full integration available - you can use steering wheel controls and the track names appear on the driver display.
Audi has near-full integration available - you can use steering wheel controls but track names don't appear on the driver display.
Of the 3, BMW is the only manufacturer that still requires you to actually use the iPod to change tracks.
Yes, I adore the 3-Series, but that's a major sticking point for me. Hopefully I can get over it ... or hopefully BMW (or Dension) will release a solution!