I have the 3G iPad and the GPS works well enough but like the comment above I wouldn't know where you would mount it short of laying it on the passenger seat. You can also get turn by turn GPS directions through OnStar.
I also have the OnStar app on my iPad that let's you lock/unlock the doors, remote start the vehicle with a visual confirmation and shows gallons of fuel left, mpg, mileage and tire pressures. Pretty cool.