GPS-- Little Function, Yet
Let's face it: GPS is cool. But the iPhone 3G's native implementation of Assisted GPS is limited, and it remains unclear whether third-party apps will do anything to change how the iPhone can be used as a GPS. (Apple's implementation uses a built-in GPS antenna, along with cell towers and Wi-Fi, to determine your position.)
I tried out the iPhone 3G's built-in Google Maps and was disappointed to discover that the app lacks audible turn-by-turn instructions. It doesn't even have automatic direction prompting, which could anticipate where you are and switch from page 2 in directions to page 3, so you can see your next step at a glance.
On a recent excursion in which I compared the iPhone's GPS capabilities with a Delphi GPS system, I found the iPhone's limitations to be quite clear. In addition to the lack of audible prompts alerting you where to turn, the iPhone lacks auto-recalculation when you change your route or inadvertently go off track.
Even the one nice feature I found was tempered by its limitations: The map and route showed where traffic jams were, but didn't provide any way to get details on what a jam-up involved or how to avoid it.
Sometimes the map would move along with the car; other times, I'd watch my car's current location move right off the map...not to be seen again unless I manually moved the map.
As it stands, I can see how the iPhone's GPS would come in handy in a few scenarios: if you're in a unfamiliar city trying to find a hotel, restaurant, or shop; if you're the passenger in a car playing navigator for the driver; if you're using location-based services; or if you're in a pinch and need some help finding your way.
The iPhone is not a replacement for a dedicated GPS unit. Whether third-party applications will step in to address its inadequacies--or whether some of the limitations are caused by the iPhone's hardware itself--remains to be seen.