I also just use the free app that comes preloaded on the phone. In addition to maps, satelite and street level view, it also tracks were you are. I've never used a GPS before but this is enough for my use. As far as paying for data, I think that you have two choices when you buy the iPhone, one of which is unlimited data. That of course costs more than the other plan. My information may be out of date.
Above all, I love my iPhone. I think it is the best gadget ever.