Among other things, the "new" unit must be 'married' to the car. Not sure how that has to happen, maybe taking it to a dealership for the software fix.
I haven't heard too many people brag up the GM NAV units either. I installed a Jensen aftermarket unit in my Corvette with a backup camera. I recently swapped out the head unit in my 09 Silverado and went with a Pioneer unit. The installer told me that the code was still set in the factory unit, so if I ever decide to go back it could be installed without having to 'marry' it again.
If you want a NAV unit, I think I would go aftermarket. JMO