Just tried using Cortana in the car, and it works great. When you play music from WP8.1 over the car stereo via bluetooth, you can use Cortana on the band without music interrupting. So for example, you can say 'play the Strokes' while something else is playing, and the music never stops. It does take a while for things to happen though.
So the big benefit: you no longer have to take your phone out of your pocket to control the music in your car. Awesome!
So the big benefit: you no longer have to take your phone out of your pocket to control the music in your car. Awesome!