I tried out the 'GPS Drive HD' app. It costs $2.99. Like Stefx points out.. It's not quite as capable as a full blown dedicated GPS unit. But it's not a bad substitute.
They give you 30 days of free voice guidance, but after that you have to pay extra to keep the 'turn by turn' voice going. You have the option of paying $2.99 a month, or $19.99 per year. The app still works if you let the voice guidance expire.
One big complaint I've seen about this app is that you have to create a unique account (with a valid e-mail address) before the app will work. Some people worry about the GPS app 'phoning home' allowing the app developer to track your every move.
My biggest complaint is that there are no internal maps. Your iPad needs to be connected to the 3G network in order to fetch the maps. If you go out in the boonies where the 3G coverage is non existent or poor, the map screen stops updating and goes blank.
My other major complaint is how the maps are labeled. If you have the display set to 'North Up'. the street names are easy to read. If you set the map to 'Course Up, then the street names appear upside down when you are traveling south or East. A clumsy fix is to turn the iPad upside down so the labels are oriented correctly
Other than that, it's not a bad app.. I have a dedicated Magellan GPS unit but I've taken to using the iPad more frequently. I really like the BIG maps on the iPad's screen.
Another thing I like is how the voice guidance integrates with the iPod app. I like to listen to music when I travel so I connect the iPad to my car radio. The GPS Drive app gives you the option of 'talking over' whatever music is playing, or pausing the song while the directions are announced. You can also set each app's volume independently.
I've been running the app for a little over a month and it hasn't given me any 'stupid' routes to the destinations I've selected.. (Where the path is clumsy, illogical, or just plain wrong).
So... It's not a perfect app. It still needs a bit of polishing up.. But if you don't need voice guidance, then you're only out 3 bucks if you decide you don't like it.