If you haven't been following them on Facebook, Twitter, or their Macworld activities.... The next iPhone app version is due out in Spring (one reply said end of March) and will further close the gap between iPhone and Navigon's hardware device features. Looks like some exciting stuff!
Some might still complain of the iPhone's smaller display (as compared to some Navigon models), but I've found it quite usable in general. My only request would be to slightly increase font sizes on some items (like the speed limit display, road names). It's not so much because I'm blind (my vision is fine), but when your iphone is at arm's length and vibrating on a car mount, it can be really difficult to read the displayed items sometimes.