The 07-08 i-drive is nice, still worth getting if it is not the 09 to current i-drive.
Many forums on this topic. Really do not use the navigation much, the screen just enhances the interior which most feel looks rather cheap without nav (for a car this expensive at least).
Having my ipod hooked up, and all my songs displaying on the screen, and being able to scroll through my tracks with the i drive knob is the main thing that pleases me. Hooking up your cell phone to the i drive with bluetooth is also nice, all calls go through the i drive instead of your cell phone.
The i drive does a lot more than nav, and in the end most don't even end up using the nav much. It's a must have for me.