Whoa... Yeah, they are a little mis-informed. That may have been the case about 6 to 8 months ago when the first GMT900's started rolling off the platform.
Not the case now... Check out http://www.pac-audio.com/default.asp
They have recently released the OS-3Bose interface. This is what you need to retain chimes, onstar, etc. It works Great! The only thing that does not work is the Avic wont fade front to back... Not a big deal in my mind. Everything else is flawless. Pretty simple install too. Plenty of room behind the heater controls for the PAC module.
I installed the bluetooth