I'm particularly excited about the Google integration as I'm a big Android fan.
Android Auto allows you use maps to navigate, hear texts and respond by voice, set a reminder by voice, play music, podcasts, and other audio sources, and make and receive phone calls. For all intents and purposes, the rest of the features of your phone are dead when connected to Android Auto. You won't hear or see any of your other notifications (email, calendar, etc.), let alone respond to them.
What does work, works well. Voice gets it right just about every time (even in my noisy truck), provided it is a voice command supported by Android Auto. My biggest complaint is if you turn the car off, you will need to re-establish your activities (route on maps, whatever your audio player was playing) when you start it again. And of course clear the friggin' nag screen every time it starts up.
Android Auto has this nasty habit of re-pairing with your phone when it reconnects after being shutdown. This makes keeping a trusted device for the NEX almost impossible.
Frankly, my Moto X in Drive Assist mode accomplishes much more, without me ever having to touch or look at the screen. Yes... Android Auto requires touching the screen for pratically anything you want to initiate, and they put the voice button in the most inconvenient place possible (for U.S. drivers); upper right-hand corner.
If I know I'll need Nav, or to search for a place on my route, I plug it in. But for short trips I don't bother. I'll just connect my phone over Bluetooth; it is damn good at auto-connecting (provided Android Auto doesn't destroy the last saved pairing).
So why did I buy it? I like toys, plus my old head unit was getting pretty long in the tooth. It will be interesting to see how Android Auto progresses. Remember this is a 1.0 version of a Google product; that's like Dev version 0.5 for other entities. The only API's devs have available at this point is messaging and audio playback. It can only get better. (c;
BTW... I went with the 4100 for a few reasons: I prefer resistive screens in the dash, I wanted the removable display, and I have no need for static maps that cost a fortune to update. And of course it was $300 cheaper, but that was not the deciding factor.
As for the non-Android Auto of it. Very happy. I have most of all my songs I would ever want to listen to on a 64GB SD card, and keep 32GB of play lists on a USB (or plug in my iPod). Bluetooth audio sounds very good, so does the phone (for both parties). I love what the autoEQ/time alignment setup did (just like in my HT), and the built-in crossover. The interface is much better than the Eclipse it replaced, but could still be better. Frustrating that MixTrax is broken on the Mac and hasn't been touched in years. And while the tag reading is good, it ignores the track number tag and plays songs in alphabetical order. If you want to hear an album in order you have to use the file setting.
Bottom line is it's a car stereo, and I have yet to find one that gets everything right, but this comes pretty close. If I had it to do all over again I think I would have done the same thing.
HTH
-SiP