Does Google Mobile/IPhone offer turn-by-turn directions for most US cities ? If so, why get a Garmin?

will Google Mobile and cell phones put Magellan, Garmin and TomTom out of business?|||Currently iPhone by default offers Google maps navigation. It is not turn-by-turn, but is still very useful and have many other features like local search that Garmin and TomTom do not. iPhone GPS is especially useful in USA. If you want turn-by-turn solution on iPhone you can buy TomTom iPhone application for about $100.