I tested the turn-by-turn feature of my iPhone and it DOES do the voice through the radio, the music is muted while the directions are given then the music fades back up. Love that feature, but it does eat up the battery life in a hurry so I'll have to make sure I bring a car charger -- the iSimple device came with a "free" car charger, but I have a better one that I'll probably use instead.

 

UPDATE: I drove my S-Type with the new TransIt Bluetooth device installed about 2,800 miles, listening to music the entire time, and using the turn-by-turn directions. It worked almost flawlessly, with one or two exceptions.

1st, the iPhone automatically tries to output the audio through the connector on the bottom of the phone which I'm using for charging only. What this means is, if I'm listening to music and I plug in the charger, the music stops coming out through the bluetooth device and I have to manually tell my iPhone to use the bluetooth device for output. This is pretty annoying, but I can deal with it.

2nd, I haven't figure out how to use this device for my phone calls, or maybe I can't. It would be nice if I could use it as a handsfree device so I could hear my phone calls through the car speakers and talk into the microphone on the phone. I'm going to work on this one, maybe it can't be done. Related to this issue -- when a call is coming in (i.e. the phone is ringing), the bluetooth device turns off and the radio plays either static or whatever is on the station I have the device set on. This is kind of annoying, but at least I know when I have a call coming in!

3rd, two times I couldn't get the device to turn on. I didn't install the on/off switch because the seller told me that I didn't need to install the switch if I used the free iPhone app. Yeah, that works for switching the radio station that the device uses, and to set the device to either ON, OFF, or AUTO. If the device won't turn ON when you turn on the radio (which it is supposed to do) then the app just displays that it's waiting for the device and won't go any further! I'm going to install the ON/OFF switch so I can manually cycle it if I have this problem again. I did resolve the problem both times by turning the radio on and off several times until the device turned on.