I thought I'd write some notes on using this for a while because no one else seems to have, and other people might find it useful.
It might not be clear from the product pages and reviews but it's important to remember - Carplay runs on the phone, it just uses the head unit in the car as a dumb touch screen, and steering wheel buttons as additional hardware. The content on the car unit is not available, indeed it runs in parallel. Use the unit GPS directions, and music off the phone or vice versa.
That means at any time there may be two different UIs in play. Yes, that is a bit rubbish. I'll admit I'm an early adopter and maybe more tolerant of rough edges and glitches. Any one one of these might put off someone with a less tolerant attitude:
Crashes, usually right off connecting the phone up. So much so, I usually reboot the phone before connecting, this avoids that.
I have an iPhone 5 so that may be less prevalent on newer ones.
Audio mix sometimes plays both car radio and iTunes. It gets confused I guess. Fixable, but annoying. - Siri is ok at best, but generally not all that great for commands. 'Hey Siri' as a command prompt rarely works. Pressing the button does. Siri weirdly works much better dictating a text message.
Physical positioning of a touchscreen, that you have to see to use, way down the centre column is... not ideal from a safety point of view. Also directions are a little sparse, so glancing down is also not ideal.
In newer, dedicated vehicles with a popup screen nearer the line of driver-sight this might be less of an issue. On the plus side: - Apple Maps, and the directions in it are quite good, better than I expected, but not as good as a Tom Tom (or the really good Tom Tom iOS App).
Sound, the head unit upgrade massively improved the audio quality over the standard VW/Audi stereo. - Speech control over text messages and dictating a (short) reply is good and genuinely cool (and safe).
Music, in sync with your iTunes, is really nice to have.
Not tried apps like Spotify, but users report many problems
Your car unit software is now tied to your phone OS for updates, so rather than improvements very rarely (if ever) regular iOS updates *should* bring improvements quicker. Or problems, as I did with 8.1.2, needing a downgrade. Ah well.
So, overall - it's interesting, better than the terrible factory radio I had but probably not for everyone.