[Android Auto General] cloudpt:
Don't know if all this info posted here already, but these are my interesting finds after 2 months of using the XAV-AX100. For all those considering soldering a 2nd USB port, read no. 1:

Important: I am still on stock firmware (not 10206).

This one is a real bother and apparently happens to other users - the unit will often reset random settings, including clock and station presets at car ignition and off (acc signal). This could be a bad installation or my car electrical spec missing someting for the unit, but this is unlikely since it happens to multiple users and settings should be flash-based (not depending on constant power). Happens at least weekly, so you better get used to the default settings and setting the clock if you have this problem.... I have recently noticed this is why AA fails to start a lot on my system (see bullet 4.): I have to setup the clock again (or wait for RDS to sync it with "Clock Time") before a phone allows AA startup on the unit. Clock is likely used for AA validation

multi-usb support does exist with a hub and without soldering, although sketchy - having a USB-flash connected to a hub, then starting a song before connecting an Android device to the hub keeps USB playback even after entering AA (might not work with all hubs and all flash drives though. I can share my setup on request). When connecting the phone, if USB is playing, you can still hear audio and go back to USB media screen from sources (volume press or using the "go to playing"). This stops as soon as you set audio to anything else (e.g. Spotify) and can only be repeated by removing the phone from the hub. I also believe that the problem people have with 2nd USB soldered ports is the same as mine - when Android Auto connects, the USB Media shortcut on home gets replaced but it's not because the firmware stopped communicating with the flash drive, it's just bad UI implementation

on a Galaxy S7 International, when removing the phone sometimes AA won't switch video to phone, triggering an unrecoverable black (off) screen that can only be rolled back long pressing (+10s) power + volume down, and using the factory menu to restart the phone (not factory reset!!!)

AA startup sometimes fails with my S7 This is likely a clock-sync issue due to bullet number 1. - roughly 1 out of 10 times connecting to stereo AA won't start and the phone will display a red screen stating the stereo is not a supported Android Auto head unit - solved by replugging the phone to USB

Apple CarPlay definitely looks sharper and more fluid, much better than AA - I believe the pixel matching is better on Apple, maybe they output a more appropriate res to the unit. Turning 720p support on AA does not appear to improve this and so there's mild blurrinness using AA

Music apps (I use Deezer and Spotify) will randomly fail for no reason at all either at startup or switching songs, but eventually recover after a minute or 2

Volume is CONTEXTUAL when using AA (don't know about CarPlay) - navigation cues, music and calls will have different volume settings and these can only be changed during situational audio output. AA music (e.g. Spotify) seems to keep the base head unit level (i.e. the same as Radio or USB volumes), but I am not 100% sure about ringer and Google Assistant replies (other than navigation cues) having a different volume context. I like it as is because this enables independant volume settings, but having a touchscreen they could have in-screen sliders for all multi-volume touch control, just like Android does in phones. This might be an AA-specific feature and not a stereo one

Volume does not adapt with engine sound. In a 2017 media player stereo with a microphone, it's tantalizing not to have this

"OK Google" is indeed only triggered by the Phone mic - you have to speak "OK Google" to the phone first, and only after hotword detection audio input will pass to the Head Unit mic - I get around this by making a habit of using the long volume knob press instead of the hotword

The unit occupies exactly 1 din on the back that goes inside the dash, but 2 dins on the front. It's sibling XAV-AX200 has a slightly larger footprint on the back (due to the optical disc reader), but still has leaves space for cabling or the (extremely overpriced and useless) GPS add-on

This one should be common knowledge, but grounding the handbrake cable (e.g. to the unit chassis itself) will prevent some AA safety checks for car being static. Do note that AA now uses GPS-based movement for stuff like list scrolling limitations

You should use the included rack for good support on your dash even if the unit fits in your single din stock bracket

THERE IS ABSOLUTELY NOT THEFT-PROTECTION ON THIS. My personal advice is to stick black ducktape on the Sony logo if your car is usually unnatended for long periods, and/or you don't have car alarm

Rear camera lines can be removed on the settings if you slide the offset to the minimum, if you want to use this as a dashcam or media player for instance

The unit uses resistive touch input - meaning single touch and no multi-touch features like pich to zoom on nav. When using resistive screens, you will get some lens distortion effect when using touch due to the resistive plastic warping . Additionally, I have cut out a plastic screen protector for a Nintendo Switch and touch works fine with it

Long-press on Home turns the unit off (sort of a sleep mode, since it does turn on pretty quick and doesn't seem to display the boot prompt; audio does stop on long Home press unlike turning screen off)

There is an option for screen off on the sources dialog (volume press), but they should really add something that doesn't require using the screen (double Volume knob or Home press maybe?)

All screen dim settings I tried seem worthless - brightness at minimum is still pretty bright for night driving comfort, especially on pitch-dark nights and roads

Scrolling through the station preset pages (which FYI you can scroll sideways, having 3 pages in total) will always change the station to the FIRST station of that set. Usability-wise, they had at least 2 much much better options, such as not switching stations when moving between pages (best option, user hasn't commit to station change yet), or at the very least switch to the last station previously played on that page

I have yet to find a way to get RDS to properly scroll or show additional song info, such as stock stereos like VW have, where large RDS data scrolls and you can also see current song playing

There is a custom fix for removing the constant boot warning. It should only be installed on the latest 1.02.06.0 firmware. Props to superdavex. Try at your own risk (I will be doing so soon)

Apparently the only supported flash drive format is FAT12/16/32 (wtf Sony). But we do get FLAC support

If buying in/from Europe, the remote control is NOT included (it still has the IR receiver)

Manual and specifications can be found here

If I had to make a list of essential things to fix on this unit, they would be:

solve the settings reset issue

make the boot prompt one-off, regional-dependant or add a setting for disabling it

add automatic adaptive volume using mic for engine sound, maybe with an offset setting or enable/disable

add TRUE USB hub and NTFS/xFAT support. It's not rocket science

sort out the bad RDS scroll and no song info, and the preset page behavior (no auto-switch)

not solvable with an update, but a unit like this really deserved anti-theft measures such as removable faceplate or a cover mechanism

All in all, I really like the unit for the price I got (290EUR, roughly ~350 USD). With all the above fixes, this unit would be close to perfection. The new XAV-AX5000 might have a wider diagonal, dual USB and Sirius XM, but I'm from Europe (no Sirius), USB hub use is very possible with the AX100, 6.4 is enough and I really preffer the volume knob over volume up/down buttons.