If you are lucky enough to be able to get a new car, then it may come with a built in satellite navigation system. These systems are usually quite good, but the problem comes when they go wrong. As they are built in to the stereo system, it may be that you cannot fix it and have to replace the whole unit instead.
If you are buying an aftermarket sat nav system then you need to think about what is important to you. It may be that you want to be able to receive traffic updates and have the system re-route you to find a quick way to your destination. Some systems come with some sort of traffic update such as many of the TomTom devices, they track the routes of other TomTom devices to see how long it has taken to get through traffic and to find alternative routes.
Most sat navs will need updating every 12 months or so and you may have to pay for these updates.