The Google navigation is alright, but remember that it DOES rely in a data connection. If you go out to BFE, you could lose your data connection and thus not get any map data.
Having maps stored on the phone would be great. I'd love it if the Google navigation would allow you to plan a trip in advance and cache the map data, I'd love it.
I would suspect a navigation app from Navigon, or other vendors, would have the map data preloaded, and wouldn't have to fetch that data over a possibly slow connection, which could speed up the app.
I've encountered in the Google app that sometimes my position on the map actually lags my real position by sometimes a few car lengths... which could possibly lead to missing a side street or something...