I have a CD changer and nav. You can't see both the map portion of the nav system while controlling the changer. Even if you use it in "split screen" mode, the right half of the screen shows coordinates and stuff, while the other half will either be the CD information or the map.
The in-dash CD still works. You press a button on the face of the display, and a motorized mechanism tilts the whole thing out of the way to reveal the CD slot behind it. It's pretty cool.
BTW, I just got disappointing news from my dealer today. I was supposed to get Sirius satellite radio installed today, and they tried but said they couldn't do it because it was incompatible with the nav system. When I bought the car a couple months ago, that was know to be a problem with the 2003 models but BMW was going to update the software for the 2004's so they would work together. I reminded them of that, and they said we tried but couldn't do it. I'm not real happy about it since they told me it would be no problem when I ordered the car. I wonder if I was duped. I'm going to call BMW. Maybe my dealer just isn't up to speed on things.