First off, the Nav. system is like $590, so depending on what options you wanted anyway, it's not that overpriced.
Second, I don't think they have a kit out for the 2013 to allow an aftermarket unit to fit, and without a kit it will be quite hard to install one and have it look decent. The kits you see that say they fit several years models including the 2013 are not going to work. The 2013 is totally different. Also the OEM Navagation has all the knobs outside the standard double din area, which is kinda nice.
Third, The Nissan Nav. also sends data to the screen between the gauges, so who knows if any of that uses the connection from the stereo, would probably be safer to go with a lower model stereo if upgrading to aftermarket and you won't get that useful feature. There's always a chance other things won't work on that screen on in the car.
Forth, The Nissan Nav. system is controlled by the steering wheel buttons, so you either need a kit or will loose that function.
Fifth, the speakers are 2 Ohm, so you will want to make sure any aftermarket system can handle that, or better yet, replace them because the don't sound great anyway.
I have the Nissan Nav. system and like it, thought it doesn't do everything my previous aftermerket NAV did. I did replace all my speakers with 2 Ohm JBL's and they sound great!
2013 Nissan Altima 3.5 SV Metallic Slate with Navigation
Last edited by westly197; 09-24-2012 at 09:20 PM.