Hey, so, have you heard your iPhone is also a really, truly good turn-by-turn navigation device? Because with the pricey-but-probably-worth-it Navigon, it is.
There's a lot to chew on in the space, and before you drop a hundred bucks on an iPhone app I suggest you take a gander at Wilson's extensive breakdown of the three forerunners. In short, though, Navigon is a dependable option, with the cleanest interface of the lot, but no easy way to get a full route overview. It stores its content locally, so data connectivity isn't a worry.
From the App Developers: With the NAVIGON MobileNavigator you can transform your iPhone into a fully functional mobile navigation system that is every bit as good as the multi-award-winning personal navigation devices from NAVIGON in terms of its user-friendliness and features.