I could see CarPlay effective/useful if the car didn't have a well designed infotainment system (Ford), but for systems like MMI and especially iDrive where iPhone integration is pretty damn good, I'd much rather use the native system than have an iOS-like overlay.
The problem with even the somewhat better designed BMW, Mercedes and Audi systems is that there's no way to do simple things like respond to text messages. With BMW you can use SIRI from the steering wheel and muddle through responding (there's no on-screen indication of what's going on).
In demoing Carplay, the coolest thing it could do, by far, was make it very easy and intuitive to have the system read back and respond to text messages. It's also easier at selecting your audio media than either i-drive or COMAND.