Other than the CD Changer drawer not opening you lose nothing.
I did not do a writeup it's very straight forward but I suggest getting a 7 day subscription to WIS and learning how to get the info you need from it.
Parts were about $4800. You need to remove anything on the most ring that is NOT NTG 2.5 (In other words EVERYTHING that is in the car stock. Radio, Audio Gateway (Amp/tuner unit in the trunk) CTEL module, SDAR module (If installed) Lignuatronic module (If installed), CD Changer, Ipod Interface (If installed) Nav Processor/DVD Drive (If installed) Bluetooth Phone interface (If installed) You replace all that with NTG 2.5 headunit (which has Nav, Linguatronic, Bluetooth and DVD Changer built in), Main Amp, Sub Amp, SDAR/HD Radio module, Media Interface (Ipod interface) and a Conversion wiring harness from Comandonline.co.uk
Install took me 8 hours going very slowly, if I did it again I can probably do it in 4 hours