When I lose navigation I just do without. I haven't replaced it with Google or any other cell-based app. The satellite radio often also drops out in these moments despite having a buffer.
Edit: Ah, I see, I meant something sort of other than that read. I've certainly had the experience of not being able to download maps, but mostly on hikes in the middle of nowhere. When I use navigation on foot in the city, hadn't had the problems I have with in-car nav.
