As a current AppRadio 3 owner I have questions about switching to a NEX CarPlay system in the near future.
1) Will CarPlay and the ability to listen to audio only work while connected via lightening cable? (does CarPlay "take over" so to speak like AppRadio does) The reason I ask is, I use a battery case, and would rather not have to remove it, each time I get into my car. I'm perfectly happy using the microUSB cable to change and having audio play via bluetooth. In the past, this has been less than ideal for AppRadio. Non-standard audio apps will play fine over bluetooth, but AppRadio ready apps go silent when not plugged into Lightning. (Pandora, MixTrax etc)
2) Is there a real benefit to having a 2nd screen any more? The new iPhone 6 Plus is plenty big on it's own, and you don't have to worry about a secondary interface getting in the way.

To point 1. Carplay works over a USB connection. What does this mean? well you have to have your phone plugged in sure, but it isn't working off of a USB 3.0 connection, or a a custom lightning adapter that requires the use of all 8 pins like the Lightning Digital AV cable. It just needs USB. So in theory, as long as your Battery supports a full USB passthrough on it's microUSB port, it should work with carplay. We won't know for certain until we actually have the software update, but I don't see how logically it could require more. However, if your battery does not use a usb data passthrough you won't be able to use it with carplay.
To point 2. Carplay is not just a system to allow you to have a bigger screen for your phone. To that yes the new iPhone 6+ not only is almost as big as the screen on your head unit, but also now runs in landscape mode for everything including the home screen. I have no doubt that other apps like spotify and beats will have updates allowing landscape view as well. However, what carplay offers is a vehicle specific interface that in theory will make it easier and safer for you to use your phone while in the car than if you just picked the phone up and used its touch screen. How good Carplay will be in its actual implementation remains to be seen.
Personally I would have liked it if apple gave us a Carplay mode for our phones as well that makes the icons easier to press and expands Siri's capabilities like carplay does based on the videos I've seen. Especially if they could put that in an iPad, then I would just buy a cheap radio with bluetooth and a used iPad mini and go nuts on my car. Alas, it is not the case.