I have had no trouble with iPhone and BMW iDrive. I USB tether the phone though, bluetooth streaming does not deliver the best audio quality.
Siri works through bluetooth, however you have to initiate it on the phone so it is not safe while driving.
I prefer the iDrive NAV to other integrated systems I have tested. I love being able to send addresses directly from Google maps to the car (from the web or the BMW iPhone App). I'm not a fan of BMW apps however.