When iOS 8 first came out, I contacted apple that HFP doesn't work as well as it used to. When using imaps turn by turn, I didn't need to have my car radio set to bluetooth to hear the turns. If imaps had something to say, the radio signal would stop for a moment, then I'd hear "turn left" and then the radio would kick back in. This stopped with iOS8. It still hasn't been fixed, AFAIK. Who the hell wants to keep their radio tuned to bluetooth for an entire ride if they'd rather hear a radio station?