Does sat-nav work? David Wilkins fixes a range of systems to his windscreen to assess them

If you've never tried in-car satellite navigation (sat-nav), you probably wonder what all the fuss is about. If you have tried it, you're almost certainly an enthusiast. Sat-nav does away with the need to stop to ask strangers for directions. It puts an end to the ritual of spending hours studying your road atlas on the eve of a long journey. It should abolish those map-reading rows that can break out between even the best of friends. It can help you save fuel by working out the most efficient route to your destination: a sales rep driving to several appointments every day could probably save a few hundred hours each year.

For anyone who is in the process of buying a new car, there is an important choice to be made between a built-in system and the much more common portable aftermarket devices. Built-in systems can cost as much as £2,000, far more than portable sat-navs, which can be under £200. Nevertheless, built-in systems have two major advantages; they are good for resale and the plumbing is a lot neater. But most sat-nav users will be going for the portable aftermarket devices of the type reviewed here; the Garmin nüvi 300, the Medion GoPal PNA 515, the Navman iCN 750, the TomTom GO 910 and the ViaMichelin X-930.

These are not strictly comparable - the pricier Medion, Navman and TomTom are the top-of-the-range models from their respective manufacturers, who also offer cheaper devices - from as little as £149.99 in the case of Medion, for example. It's also important to stress that, because they all use the same basic location-technology based on the US military's GPS satellites, any of these sat-navs will perform the basic job of getting you to where you want to go fairly satisfactorily. The main differences are in terms of extra features - so be prepared to spend a lot of time looking at specifications. Cheaper sat-navs for example, tend only to cover the UK, while the pricier models often take in the whole of Europe.

There are three important things to look out for. The first of these is the method of inputting your destination. A sat-nav system that allows you to input a full postcode will save you a lot of time and is more likely to get you to your precise destination. The TomTom and the Garmin, for example, use full postcodes, while the cheaper ViaMichelin does not use the latter digits, so the street address has to be input as well.

The second is general ease of use. I attempted to set up and use each of the devices without consulting the instruction manuals. All passed this test, although I accidentally selected the wrong world region during the set-up process with the TomTom and it took a fair amount of fiddling around to rectify this. In general, though, the TomTom scores well in terms of ease of use but I would single out the Garmin as the best in this respect. Simple menus help, and the screen graphics are very clear too, at the expense of some detail. This is probably the one to go for if you are drawn to the benefits of sat-nav but daunted by the technology.

The third thing to look out for is the method of mounting. Most aftermarket sat-navs are attached to your windscreen via a suction pad and a mounting bracket, not always reliably. I was particularly impressed by the Navman iCN 750's industrial-strength bracket which had a plunger-style collar arrangement that generated far more suction than any of the others. But this was offset by the fact that its power cable came adrift very easily.

Another feature to look out for is dynamic route recalculation in response to traffic conditions. All sat-navs will quickly recalculate your route if you disobey their instructions but some, such as the Medion, can pick up traffic information from TrafficMaster and rejig your route in order to avoid jams.

The TomTom is the device that has the strongest word-of-mouth support. It has the funkiest design and it's the one everyone has heard of. I suspect that people will probably talk in future of TomTom-ing their way to destinations.

The Medion's shiny casing doesn't have as much dashboard appeal, but hides a capable device with a lot of features for the money.

The ViaMichelin, as befits its lowish price, doesn't have some of the fancier features but instead offers something different, the ability to locate "points of interest" based on the Michelin Green Guides. This is the one to buy for golfers and gourmets.

Some bells and whistles have little to do with navigation - the Medion can act as an MP3 player, store digital photographs, and act as a Bluetooth hands-free kit for your mobile phone. The Navman incorporates a camera and the TomTom has lots of extras too.

So what about all those stories you hear about sat-nav sending drivers down impassable farm tracks or shoving huge volumes of traffic through previously peaceful villages? This is rare. Instead, sat-navs can save you time, money and frayed nerves.

How do these guidance systems compare? A buyer's route...

Garmin nüvi 300

£256.46 at

For: Superb ease of use

Against: Lack of screen detail

Medion GoPal PNA 515

£349.99 at Halfords

For: Feature-packed, great value for money

Against: Not exactly dashboard eye-candy

Navman iCN 750

£429.96 at

For: Serious piece of equipment, excellent mounting

Against: Heavy cable comes adrift easily

TomTom GO 910

£499.99 at

For: Design, the one to have, nobody will laugh at it

Against: Some offer more for the money

ViaMichelin X-930

£234.99 at

For: Tie-in with Michelin's guides, low price

Against: No full postcode input

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