I had no issues with bluetooth with ios 8 and my iPhone 5. My iPhone 6, it went to hell. I've got a Nissan which has its own built in address book, and figured out if I dialed through it or received a call, it worked, but if I attempted to dial from the phone or Siri, no call audio would go through (but the phone would say it was in bluetooth). Siri, voice nav, anything else would transmit via bluetooth just fine.
Once I updated to 8.0.2, it's gotten better. Now, if I initiate a call from the phone or Siri, while it immediately disconnects the bluetooth, it hands it back off within a few seconds of the person answering.