As someone who has had the Nissan/Infiniti CD based NAV system and currently has both the DVD based system (in a Q-45) and a Garmin StreetPilotIII, all I can tell you is, go for the Garmin SPIII. Current street price is around $700-750 with a full unlock of the entire USA. It's portable, has a great color screen (not quite as large as the Nissan built in) and has voice prompts for turns. Compared to the $2000 (retail) Nissan system, the only thing you lose is the "Birdview" screen display. Both have to be turned on manually (the Nissan system requires a keypress after it boots, the Garmin requires you to manually turn the system on). The major advantage to the Garmin is that it can be used fully while the vehicle is in operation, while the Nissan system keeps you locked out of many features unless parked, or at a complete standstill. The Garmin has a better database of POI's, especially restaurants and will a LOT cheaper to buy the updated DVD's. The major advantage to the factory system is that is IS built in. but that also means you lose when you unload (sell) the vehicle (or are in another car, ie: rental, 2nd vehicle, boat).
When my current Pathfinder goes away in December, the Armada I'm replacing it with will be ordered without the factory NAV system. I actually prefer the Garmin, and it's paid for.
Chuck