Jeff Behm explains the iPhone's usefulness – Jeff works in an all-Mac studio environment, and the iPhone syncs his "… calendars, contacts, email, web bookmarks, and many other applications – these automatically – others with very little effort."
Regarding GPS and Google Maps, Jeff adds:
With the included GPS and Google Maps, yesterday I drove right to a location scouting site with zero glitches and no computer mapping. Just needed the address. I can find my location at any time on Google Maps with the GPS, since where I am is displayed in real time when the Location signal is turned on.