I have the 660. My bike also has the full radio package. As has been noted by hschisler and Sleuth, the Nav IV has no stereo audio out except via BlueTooth. If you wish to use the Nav IV mp3 player, that can be a problem if you are already using the A2DP connection in your headset to pair to the bike's radio.
With the 660, though it requires a bit of work and a $25 power connector to install, it's very easy to run the audio out cable into the AUX port and listen to the mp3 player. This is a nice feature given that, over BlueTooth, the FM/AM and XM audio quality is poor. The USB and AUX audio quality, mystifyingly, is very good.
If you want to have the BMW Motorrad Dealership POIs on the 660/665, you can download an executable file from Garmin that will install them on either unit very easily.
The Nav IV is a rebranded 660 and the firmware is probably the same. There is another, secret, bit of firmware that allows the unit to be controlled by the four buttons in the cradle. I traded an extra car cradle for a Nav IV cradle and if I ever get my paws on that bit of software I'm going to risk bricking my 660 to see if I can run it with the cradle. Then I am going to hack the cradle to get audio out. The chances of me getting a hold of that firmware, though, are Slim and None and Slim just left town...
Tom