Using GPS software on your iPhone has both benefits and drawbacks.  Navigon MobileNavigator is aided greatly by the iPhone's hardware.  Once you start your GPS app, your iPhone will connect to a satellite almost immediately.  In all of my tests it has taken just a few seconds before a satellite signal is found.  In my previous experience with GPS standalone units I have never gotten a signal even remotely close to how fast the MobileNavigator does.  There have been times where I would be sitting in my idling car waiting for a satellite signal, at times taking several minutes.  This speed also translates over to when you veer off course.  The MobileNavigator doesn't take very long to recalculate your new course.  In fact it is done instantly. 
Of course with all the benefits of using MobileNavigator, there are some drawbacks to having a GPS on your iPhone.  First of all because it is running on your iPhone, it will drain your battery considerably faster.  In fact using GPS is one of the most draining activities you can do on your iPhone comparable to recording video or playing a 3D game (as reported by the Battery Doctor Pro app).  Another minus to having GPS on your phone might be how clumsy it is.  You may not have a phone holder set up in your car so you may have to resort to holding it in your hand, throwing it in your cup holder or resting it on your passenger seat.

Feature Rich and Easy To Use, Even For The Not-So-Tech-Savvy

Which route to take?  The choice is yours.

One thing that annoyed me with basic GPS systems is that I have very little control over which route I want to take.  To have any say in which roads to traverse it usually had to involve changing default settings in the GPS, configuration to avoid toll routes, highways, etc.  When you plot a route with the MobileNavigator you are given up to 3 separate routes which you can select from.  If you feel like taking the scenic route or simply want to avoid highways/freeways, MobileNavigator makes it as painless as it possibly can be.

Searching points of interest is as equally painless as selecting your routes.  When you go into the POI menu there is a button for gas stations, parking and restaurants.  Selecting restaurant, for example, will display all restaurants starting with the closest one and show the distance.  If you select one of these restaurants, you will be provided with the phone number which you can call with a simple tap.  You can also view the weather for the immediate area, the map of the restaurant, other points of interest close by and of course have the GPS direct you there.  If you have something more specific in mind you can search your POI by name or category as well.  Now all of these features are dependent on how good the POI database is.  Living on the outskirts of a major city I can say that the POI database is exceptional.  I can find directions to even the most remote mom & pop shops as it seems every business I pass is in the GPS.  Of course this is only my experience with it and your experience may differ.

Craving a Big Gulp?  MobileNavigator has you covered!

Turning your phone sideways shows a wider view.

Cons:

• Drains phone battery faster
• May need to buy a car phone holder and charging kit
• A bit pricey