I have two cars. Audi S7 and BMW X5.
On both cars, the basic functions (with my Androids) work well, such as contact syncing, taking and receiving calls, but the problems occur during A2DP streaming. It's slow to respond when I switch the track, or it just doesn't work. Or the meta-tag data doesn't update on the MMI/iDrive screen.
Now if I plug in my 5S to my X5, I get album art and I can control Spotify, etc from the iDrive controller. Same with the Audi. The integration is just superior.
Now I'm sure once CarPlay and Android Auto take off, all of this will change. Audi is one of the first makers to sign up for both CarPlay and Android Auto. I'm excited to see what the future holds.