I thought about the same thing. It is very difficult to get the same level of integration if you put an aftermarket Nav unit in place without replacing everything from the radio, to the display, CD player, etc. Big $. Seems like a car computer is the end solution, but there are no turn key setups so unless computers are your hobby this is a huge effort. Big $ there too.

So, if you value things like: working steering wheel controls, radio mute for nav directions, MPG calculations, factory appearance, etc. you're better off with the OEM solution. Especially since you already paid for it!