The built in sat nav isn't the most sophisticated, but it always gets me where i need to go to. Using Car Play or Android Auto isn't without its drawbacks;-
You can't run Google Maps or similar in the active info display.
You can't have two mobile phones connected at the same time as App Connect is running. Not a big deal to some, but I have my personal and work phones connected while commuting.
You can't listen to music through anything connected via the aux while App Connect is running. I have an 80gb iPod Classic connected.
App Connect isn't wireless. You have to take your phone out of your pocket each time to connect it to the car. And then remember to disconnect it again when you've finished.
On balance, i'll stick with the built in nav, at least until App Connect becomes a bit more advanced.