You have two main options when it comes to GPS apps for the iPhone: apps with offline maps and apps with online maps. Examples of apps with offline maps are the Navigon, the iGo My Way, TomTom, or the recently added Magellan RoadMate.
These apps are excellent for frequent users as they don't require a live data connection to work. All the maps are included with the app and downloaded to the phone. However, they tend to require gigabytes of storage space and take a long time to install. If you plan on going on a long road trip, they are good fits.
If you are a casual user, however, it's better to use an online GPS application. These applications are just a few megabytes in size and therefore take a few seconds to download to the phone via a 3G connection. This means you can immediately get one the moment you suddenly need turn-by-turn directions.
The first online GPS app for the iPhone is the AT&T Navigator, which works pretty well. Unfortunately, it's only available to AT&T customers and is rather expensive ($10 per month) for what it offers. The good news is, you now have other and more flexible choices.
I've been trying out two alternatives, the Gokivo Navigator and MapQuest Navigator. Both of them require a live data connection to use but they can be used on any GPS iPhone (including jailbroken ones), meaning you don't need to be an AT&T subscriber to use them. And there's at least another reason you want them over AT&T Navigator: they are much cheaper.
MapQuest Navigator costs just $3.99 per month (or less if you pay for a longer period) and is a simple navigation application. Once launched (it takes just a second or two, by the way), you will be greeted with the live maps and a list of popular points-of-interest (POI) categories. You can either pick one of those, type in an address, or pick one in your phone's contact list. After that, the navigation is straightforward. The rerouting is not, however; if you miss an exit, the app might take a while to reroute, and if you are on a freeway you might miss another exit by the time the rerouting is done.
Costing another $1 per month, the Gokivo Navigator offers a lot more than MapQuest. The app had a shorter rerouting time during my trials and it has integrated iPod controls (so you can manage your music playback without having to exit out of the app). It also offers a friendlier interface with much more viewing and searching options than MapQuest.
Both of the apps share a long list of useful features, including integration with the phone's contact, and text-to-speech that speaks out the names of the street or freeways, and real-time traffic. The real-time traffic on both applications worked well overall, though sometimes road conditions weren't updated fast enough. However, this goes the same for most real-time traffic applications, including Google Maps.
The real-time traffic feature of Gokivo also offers the detour option when there's congestion ahead. With the MapQuest app, you will have to manually reroute to avoid traffic. Gokivo also allows for dialing up a POI's phone number, which comes in handy in case you want to make sure the restaurant or a gas station is still open before getting to it.
The accuracy of maps on both applications is on par with that of most GPS apps and devices I've used; they both seem to have very up-to-date maps. During the trials both of the apps were able to find the addresses that some other GPS devices failed to do.
On the other hand, both of them also share the same drawbacks. The first shortcoming is the fact that they have to rely on a data connection to do every task, including downloading maps, routing, looking for POI addresses, and so on. In areas where there's no cell signal, these apps are useless. In my experience, they also don't work very well in big cities where the wireless signal is choppy. However, this is not the apps' fault, and the experience would vary depending on the cell service providers.
The second big shortcoming is that neither of these apps can work when the phone is in a horizontal position. This is not a big problem when you are on a freeway, but when in town, it limits your view of surrounding streets. The two also suffer from what I call "one-track navigation," which forces you to first exit out of the current route before you can go to a different address or even change the viewing options. All the offline GPS apps I've reviewed allow for quickly finding a cafe or a gas station in the middle of the journey, then resuming to the main destination.
I also don't like the lack of a lane-assist feature (which tells you the proper lane to be in, say, when exiting or merging), and there are no routing options for pedestrians or public transportation.
All in all, however, either of these applications will turn your GPS iPhone into a decent navigator. Personally I prefer Gokivo because of its many other advanced options and the friendlier interface. MapQuest, however, is $1 cheaper for the monthly plan and is even cheaper than that if you pay for a yearly plan.
The best thing about these apps is the fact that they don't require any commitment and, again, that you can get them instantly. When you are lost and desperate, having either of these apps is worth the price.