I also use the TomTom App on my iPhone and am happy with it. Since it has a full basemap on board, you never miss a turn because the data network is slow. It takes about 1.4GB of space on the phone, which is fairly large, but I much prefer having a "real" GPS mapping solution in hand, especially when I spend so much time in places with really poor mobile data coverage (that would be New York City and Los Angeles).