If you have an iPhone 3G or 3GS and want turn by turn GPS directions, you may want to take a look at Sygic Mobile Maps. They are having a sale for the US maps @ $39.99 USD and North America maps for $59.99 USD.

With the latest update, they have really taken some time to refine this app, and the new text to speech feature is very good from my preliminary testing.

If you aren't looking to spend the $99 bucks for the TomTom app, Sygic is pretty good and gets the job done at a much lower price.