I've had it in the last two cars of mine. You can set it to activate automatically when you plug in a iPhone or manually so that you select the activation from the telephone menu in COMAND.
It's limited in functionality, to be honest. If Car Play is activated you can't use the COMAND navigation, you have to use Apple Maps which in itself is pretty poor. You can't listen to music from any other source other than your iPhone (so nothing off SD cards or usb devices) but you can choose radio.
You can send and receive text messages using Siri, so you can't read any text, just hear the messages being read and you can dictate replies.
You can use a basic form of Spotify which actually works quite well.
I really only use it to send (dictate) texts but even then not very often. Not a great addition and I'd not miss it.
Steve