Of all the GPS apps I've tested, I found the Navigon app to be the most full-featured and accurate. The screen is very clear and it is easy to use the many features included. I especially like the display of speed limits, and the ability to get a warning if I go over. Text-to-speech is very clear, and local search, which uses Google, is very handy and keeps the points of interest information up to date.
You can also enhance the app with additional purchases of live traffic info, and a 3D panorama view that shows you the contours of the geography you are driving through.
My only quibble with most of the full featured nav apps is that they are too darned expensive. The Navigon sale price puts it where I think the retail price should be all the time. At the upper end of $80 or $100, it puts these apps too close to the versions that have dedicated hardware.
If you've been lusting after GPS, it's a great time to grab this best-selling app. The sale lasts until June 11. If you've been tempted before, but the price put you off, you've lost that excuse. Last November, Navigon cut the price of the app to $69.99, so this is the best price yet.
Navigon MobileNavigator runs on the iPhone, and iPod touch with additional hardware. A company spokesperson told me they are considering some specialized iPad apps but have nothing specific to announce yet.