I just installed my Carphonics CarPlay unit and happy to share some experiences.
Firstly, dealing with Baris was great. He communicated clearly, and was very knowledgable and helpful in recommending how to exactly configure the system to your specific car and options. Also dispatching the unit went as promised (I picked it up during a visit to London).
Regarding my car, it is a 2003 E85 Z4 with OEM navigation, Bluetooth and TV tuner. A previous owner also installed a Bluemusic unit connected to the CD changer for both streaming and phone calls.
Bluetooth connection
Baris recommend in my situation to use the existing OEM Bluetooth for phone calls. That offers the advantage that it works in all modes, whether you are using the factory system screens (eg radio, menu settings etc) or in the CarPlay mode. An incoming/outgoing call on the OEM Bluetooth will pause/override any other audio source (radio, CD, streaming) being used. Whereas as I understand a call on an aftermarket kit or through the CarPlay will not interrupt for example when you are listening to radio.
He did recommend to upgrade the Bluetooth module to the latest Gen 13 version for most stability and compatibility with most phones. I picked one up from a breakers, and it was a simple box swap. And of course changing the Bluetooth connection on your phone, so you connect to the changed Bluetooth module. I believe you can code the Bluetooth module to have the same Bluetooth ID and password as your old uit, but I didn’t bother. It is a bit of work, requires NCSExpert I believe, and as both ID and password are printed on the module you can just use that information. At the same time I also changed the original (green) OEM microphone at the rear view mirror for a later and supposedly better version.

The ’new’ Gen 13 Bluetooth module.

Bluetooth module location. Remove bracket and it is easy to swap the module out.

The old green type microphone. The newer types are better and more clear apparently. I have no complaints of people on the other side of the line.
Baris informed me that for the Z4 he recommended using a Bluemusic dongle connected to the CD changer for audio streaming. With me using the OEM Bluetooth I could however not use the already present Bluemusic call & streaming unit. As that meant the phone would ‘see’ two Bluetooth carkits, both the OEM and Bluemusic, causing issues. So he supplied a Bluemusic unit for streaming only. This way the phone uses the OEM Bluetooth for calls, and the Bluemusic for audio streaming (most phones can support two or more different Bluetooth connections). So I simply swapped over the Bluemusic units and job done. Also allowed me to remove the microphone from the previously installed Bluemusic, cleaning up the interior.
Head unit
Baris also recommended for pre-LCI cars to install the head unit of the later LCI cars. This as these are better compatible with the CarPlay unit. These later head units can be recognized by a ‘gold’ finish at the rear of the unit. I picked one up from a breakers.
The LCI head units however have black face plates, whereas the pre-LCI have a grey colour. So a swap of the face plate was needed, quite easy, just a bit of figuring out what parts need to be swapped. What caught me out that on the reverse of the face plate is a metal mounting plate, that also needs to be swapped. So now I have a LCI had unit, a LCI face plate mounting plate, and a pre-LCI face plate.
Oh yeah and getting the center vents out to remove the head unit was a pain. Probably the thing that took the most time of the whole install...

Checking if it all works, haven’t swapped the face plate of the LCI head unit yet.

Swapping the face plates.
TV tuner
The actual guts of the CarPlay unit are in the modified TV Tuner. As I already had one in the car, it was just a simple swap of the box.

As I didn’t make a good photo, hereby a photo from the forum of the location of the TV tuner.
Using the module
As others have described and shown, it is a great integration of CarPlay in the OEM setup. The combination of the CarPlay unit, OEM Bluetooth, and Bluemusic streaming dongle works great considering it is several systems kind of tied together.
Best way to see it in my case is having factory handsfree calling capability running next to an audio input, either FM radio, a CD, or streaming from your phone through the Bluetooth CD changer dongle. This all works also without using CarPlay. The CarPlay is a sort of overlay that allows control of all of this, and gives access to apps (maps, spotify etc).
Drawbacks to the system is as it is kind of tied together that controlling is limited by the available hardware, so no touchscreen or easy controller. Carphonics have done a great job of making the OEM knobs and buttons work, but is of course not as tightly integrated as a modern infotainment system. Secondly you are limited by the screen, which of course is 20+ year old tech. So resolution is limited but still very usable. And I find the colours a bit ‘washed out’, but that might be some more fine tuning of the settings. Or maybe my particular screen is on it’s last legs...
All in all, my Z4 is my only car, and I also use it for business purposes. So I’m looking forward to using the system, and have a bit more up to date infotainment and navigation available in the car. Whether for a nice Sunday drive, or on a boring commute.
Also I love the OEM look of the car. Honestly, I think there are options out there such as the Viovox system that are a more modern take with a bigger and better (touch)screen and keeping OEM integration. But that means sacrificing the OEM look, and I find the bigger screen a bit intrusive. And living in a big city a big screen planted on your dash is always a target for a break in, heard enough people getting these screens ripped out.
Some further notes. I ordered the unit with the optional reverse camera option (to be installed when weather is nicer), but on installation I noted that my Z4 already had what looked like an aftermarket cable adaptor for that. So maybe check if your car already has one, otherwise no need to add it.