What so many products claiming to be the best it's hard to choose which product to buy. It all depends on what you are looking for. No need to pay extra for features you don't really use anyways. All GPS systems are free with no montly fees to use like satellite radio. However some products offer optional extra features such as traffic updates, weather, and other info for a fee of $5 to $10 a month. This is not require to use the GPS system it is just optional services you can add should you choose to utilize these extras services.
Some features that are available are traffic updates. You get traffic conditions so you can avoid certain routes to say you time in case there was an accident or road closure. Some GPS systems have this traffic compability built in others you have to buy an optional antenna.
Look for systems with quick startup time in regards to locating satellites. Some systems are much slower than others so look for fast ones. Usually the more satellites a system can pick up the faster startup time it has. Most systems can connect to 12 satellites but some can connect to up to 20-32 satellites.
Look for systems with easy to upgrade maps and the most up to date maps. System that have maps that are even a few years old will not see new roads and developments and also have outdated POIs such as locating a restaurant that is no longer in business.
Make sure the system you buy has the Sirf III chip in it because this is the latest chip technology with the quickest satellite locator. Almost all new models made have this chip built in. Stay away from models that don't have this chip as it is old and slow technology.
Portablity can be useful if you plan on using it outside the car as well like navigating on foot. Be sure if you are planning on using it away from a power source that it has decent battery life. Most will last anywhere from 2-4 hrs and some even up to 10 hrs. If you only plan on using it in the car then battery life is not very important.
Make sure that the volume is sufficient when you are driving or it can make it very hard to hear directions when you are driving especially at high speeds, radio turned or or people talking. Some units have good powerful built in speakers while others have a built in FM transmitter or a transmitter in the docking unit so you can listen to it through your car fm radio. Some have headphone jacks you can plug earplugs into. If you plan on using the headphone jack never put on both wear both earplugs because it is illegal to wear both at the same time as you need to be able to listen for possible approaching emergency vehicles.
Some GPS systems can even play DVDs, MP3s, videos, and XM radio. For the XM radio function you would most likely need to buy an XM antenna as well. One really nice sleek modern system is Alpine's Blackbird. It plays MP3s and is very small and has a ton of features. Polaroid makes one that has a built in DVD player. Many systems allow you to play MP3s and bluetooth capable units allow you to make hands free calling in your car. Sony make the NV-U70 which is said to have one of the best razor sharp graphics compare to the competition this is according to some of the reviews.
Most systems use a 3.5" touch screen to navigate. However a lot of the newer models are now using 4" screens such as the Tomtom 910. However there is a screen size in the 5", 7" and even 10" ranges. Some people like lots of features and some people just want a simple and easy to use product. So do a little research before you spend your money as many units start at the mid $300 to upwards of $1000s.
Also pay attention to mounting options for your particular unit as where you put it is also important and can be illegal in certain states. In California it is illegal to mount it on your windshield because it blocks your view. However you can mount it on your dashboard, air vents, cup holder, or even floor mounts. Make sure the unit you get has many options.
Look for screens that are transflective. These screens help you see better with the bright sun. The sun makes it very hard to see many screen so if you can find one with a transflective screen that is a good idea. However there are many screen hoods being made for GPS units and PDAs that helps fight this glare problem. Many Dell Axims and HP Ipaqs have transflective screens.