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.