We use it overseas (Greece, UK, France), initially because the charting was 1/2 the price of Navionics. But now, the current version of the app is VERY GOOD and stands with iNavx and Navionics. As Mark noted, the method they used for positioning the cursor for marks or distance measuring was funky but now is way better.
The next version due very soon will have AIS over cellular and real time and forecast weather.
In addition, if you buy if for the pad you can run the app on your iPhone as well.
I'm a fan. But also look into NobleTec TimeZero.