This apparently doesn't work for everyone, but it did for me. I have the iPhone 6, and a 2013 535xi. Everything works fine and as expected (handsfree, calls, audio for music, driving directions over speakers, etc).
1. Upgrade to the latest iOS (8.0.2), and the latest software on the BMW (www.bmw.com/update)
2. Backup your iPhone (just in case, but there's no restore needed)
3. Remove all paired phones from your car (delete them from the Connections profile)
4. Remove the paired BMW from your phone (delete it from Bluetooth settings)
5. On your phone go to: Settings-General-Reset-"Reset All Settings" (see below)
In my experience this basically removes any configurations from your phone, but you don't lose any data, apps, pictures etc. For instance, wifi connections needed to be re-entered, wallpaper reverted to default settings, ring tones went back to default, etc. Basically any tweaks you've made will need to be made again which is a pain, but also a good way to familiarize yourself with some of the new tweaks available in iOS 8. Also, this apparently resets the bluetooth configuration beyond what you can see, which is what is required to make this work again.
6. Go back to your car and re-pair the phone. Everything should work.