For the occasional user - maybe.
For someone who uses Nav more frequently probably not, mainly because of the data usage issue - with caps it could easily eat into your data limits; plus you're tied to having good data connection as you travel.
What's really interesting is not the Nav app but what it portends in light of Apple's agreements with car manufacturers. This is just a step towards Apple offering their own version of OnStar - on steroids. Imagine a Siri based vehicle communications system that integrates navigations, delivery of entertainment, provides recommendations and answers quires, reads and replies to text via voice, and also is a phone. The vehicle manufacturers will provide the data pipe and Apple the content and apps. CarFi would let you use an iPad as well through the system - who needs a DVD player built in? It's not that far off...