Being able to handle text messages and phone calls far better than the NEX could do on its own. While apple maps isnt as good as Waze, the interface and responsiveness is far better than waze was in the car. I quite like having control of different music apps from the radio. Phone calls sound nicer lately although they didn't at first. Having buttons in siri especially when handling messages is a god send.
I don't like the album art or lack thereof but that's pioneer's fault for putting in a uselessly bad display. I wish apple maps had live rerouting based on traffic. Besides that I really like carplay. I use it every day and it is the only reason I haven't tossed my 4000 because as a radio that radio has left me unhappy, especially since it's less than a year old and pioneer is no longer supporting it. However apple will support carplay for years to come so it will only get better from here.