I'll just chime in to say that if you keep the car for at least five years, it may be to your advantage (in terms of resale value) to have no Nav/iDrive at all. Nobody who buys the car from you then will want to have the "old" iDrive. I've got an 06, and I'm kind of glad I don't have the iDrive. I don't miss it. But then again, I bought used.
Edited to add: this may be me rationalizing my lack of iDrive! ha ha ha