Kicking Tires takes the iPhone's first turn-by-turn GPS navigation app, XROADS G-Map for a spin, and while it's got some issues, it's an "outstanding value" for $US40.
Here's the major issue, which is worse if you live in Illinois: The app is a 900MB beast, so it's split into east and west sections of the US, which have to be purchased separately at $US20 a pop. The funky dividing splits Illinois into north and south halves, with each map app getting a piece. The other bad part is that not every street is labelled, so you actually have to use the turn-by-turn sometimes, which doesn't have any voices to guide you.
But everything else is pretty gravy: It has "very clear, accurate and understandable direction," sweet 3D renderings of highway interchanges, an easy-to-use UI, awesome point-of-interest functionality, and it comes with a decent car mount. Bottom line is that for $US40, it handily competes with a $US200 dedicated navigator. So be scared Garmins and Magellans of the world, be scared. [Kicking Tires via Ray Wert's Go-Kart Blog, iTunes]