Good point on the Nissan nav. I have never seen any of Nissan's nav products. Time for a test drive...
Also, you made me think of one other killer feature on the BMW nav system-- seamless integration with the phone. So far, I have not seen another system anywhere that does this. Say you search the database for a restaurant. If you click on the phone number from the nav screen, the computer will dial your phone automatically. How cool is that. I have used that feature on road trips a number of times.
Last, you are right about the iDrive thing leading the world. The one place where BMW probably got beat there is Audi's MMI. I test drove an Audi A8 last week, and their MMI smokes everything else on the market. The key is that they surround it with several function buttons (Radio/Nav/Setup/etc.) to avoid a number of knob clicks, and the system has a "Back" button. A buddy of mine with a 550i loves his iDrive, and his single complaint is that it needs a Back button to be perfect.