Kind of depends what you want to know, really......
I got one recently but illness and then a bout of ice 'n' snow here means that I haven't fitted it to the bike yet. I've used it in SWMBO's car on battery power because it doesn't come with a car mount. In that mode, I noticed that running alongside the nüvi 770 that she has in her car, the Nav IV, really just a Zumo 660 in a posh dress, didn't show as much road detail at the same level of zoom as the nüvi.
The Nav IV mount which turns a Zumo into a BMW Navigator disables a couple of the Zumo functions by obscuring the MCX connector for an external antenna and the 3.5mm jack for audio out, while the unit is in the mount. The power cable for the Nav IV is hard-wired into the mount and the other end has the dedicated connector for the BMW loom. The corresponding connector is located close to the oil cooler on the GS. The other function on the cable is an audio out but I believe that this is intended to interface with the sound system on R1200RT. There is no other wired audio out. The intention is that the Nav IV should pair with your Bluetooth helmet. There have been reported difficulties in pairing the Bluetooth of the Nav IV with aftermarket systems such as the Autocom Bluetooth adapter.
Anything else....?
HTH