I have a new Audi which comes factory prepared with Bluetooth. At the moment I use a Nokia 8910i in the car and it works jsut great.
I've received a Samsung D600 that doesn't work so well.
The Samsung pairs with the car OK and can be used for calls but the quality is poor. Also (unlike the Nokia) I can't access my phone book, signal strength, recent calls and all that good stuff.
I realise OEM (Audi) equipment can be a little quirky but the features are all supposed to be supported.