GPS units like those made by TomTom have a number of key selling points that make a dedicated device king of the crop when it comes to getting from A to B and beyond.

A large screen, good GPS receiver, the ability to search for addresses and points of interest all at the press of a button. However as the iPhone promises all of the above can it really save you having to worry about a dedicated unit?

Before you think that the app is going to be 59p or the like, it's not. This is a full blown GPS software package and because of that it will set you back £59.99 for the UK and Ireland maps app and £79.99 for maps of Western Europe. You get the application on your iPhone to use as you will in your car, but you don't get the optional iPhone cradle you might have seen.

Structured like the TomTom software on the company's dedicated units, the main menu screen provides you with a number of navigation and setup options. Settings include enabling sound, night colours, 2D map, Manage favourites and other not so exciting options. Yes you can change the voice, language and tone, but TomTom hasn't bundled anything out of the ordinary here. UK still gets Jane and Tim, although opting for Ken from Australia or Katrina from New Zealand will give you that holiday feeling.

The settings also allow you change distance units, route favourites like Toll Roads and whether to avoid unpaved roads. Get past the settings and you get a number of features: you can call a POI, browse the map as you would a Google Map and plan a route ahead of schedule.

The Advanced Planning mode is incredibly handy as using IQ routes (included) you can work out how long a journey will take at a specific time before you even get in the car.

For those in the dark as to what IQ routes is, it's basically TomTom's massive database of traffic and scheduled estimated journey times. Rather than saying it will take you the same length of time to cross London at 5pm on a Friday and 2pm on a Sunday, it knows that the former will involve heavy traffic and gives you a travel time accordingly.

Before you press the route button you are given a series of choices from: fastest, shortest, avoid motorways, walking, bicycle, limited speed and choosing gives you different results.

Driving from Tower Bridge in London to Edinburgh Castle leaving at 7am will take you 7 hours 15 minutes for example while walking will take you considerably longer, 122 hours.

Of course the main function of the TomTom for iPhone app isn't Advanced Planning but day-to-day navigation and for that there is the Navigate To... menu option. Here you can punch in your Address, Recent Destinations, Points of Interest, Postcode, Point on Map as well as Contact from your Contacts Book on the phone.

Full postcode support is provided allowing for quick address entry and it was the most common way for us to enter an address in our tests. Entering information into the system is all down via the touchscreen interface, and the application works in both portrait and landscape modes making it easy to enter information.

So the data entry is easy, the software offers plenty of features, surely this will replace the dedicated device market? Not so fast.

The software does work very well and we had no qualms with the software, the GPS signal or the performance of the device. The TomTom for iPhone app was quick to start and quick to find a GPS signal in our tests. Likewise traffic updates while driving were as good as a dedicated machine.

So what's the catch? The main one is that you've got to find somewhere to perch it on your dashboard, the second is running the GPS on the iPhone is a power hungry experience. The iPhone battery is poor as it is, let alone if you plan to drive for 2 hours on top of that. Just don't expect to get much use out of your phone at the other end if you've stuck to the battery.

Of course there is a solution. TomTom just happen to make a cradle, which for an additional £99.99 will offer you a place to put your iPhone, will boost the GPS signal and offer you a speaker to boost the audio as well.

But here is the rub? £99 plus the £59 for the app means that before you know it you've spent £158 for a device that is as good as a dedicated device, but not a dedicated device. You'll still have the cradle to hide and you'll still be able to get cheaper solutions, even from TomTom themselves.