Just to add my experience with an aftermarket HU...I was able to change the oem and put in an alpine cda117 that I had from another car. I mainly did it for bluetooth and Ipod.
My car had the Hi fi system, and I knew from reading the site that I would lose some functions ie: PDC (which I dont have) the ability to have updates done through the OEM HU, and the chimes.
In my case i didnt mind the updates...if i ever needed them I can switch the radio out in ten minutes. But I expected to lose chimes and didnt! I dont know why, but i'm ok with that. I was also able to retain steering wheel controls.
So FOR ME it wasn't as bad as everyone made it to be.