You can now buy a GPS navigator app or application for your iPhone that claims to do almost everything that the stand alone GPS navigators do. The question is, are they worth the money. Most iPhone GPS navigator apps sell for under a hundred bucks. You can get a GPS now, a stand alone one, for about that much.
There are some serious problems with using your iPhone for your GPS navigator. Among these are the fact that the iPhone is not a GPS. It has one tiny little antenna inside the case and will not pick up or lock onto satellites as well as a standalone GPS. Another problem is that when you want to use the phone you leave the GPS navigator mode, making it hard to talk to someone who you are trying to find, and find them.
For more on how well iPhone GPS navigator apps work see: Are iPhone GPS Applications Worth The Money?