The quality of the GPS signal (reception) is obviously paramount. Opt for a device using the SiRF Star III standard.
The screen quality and graphics. You will need to check the following features: screen size, brightness, image quality and anti-glare effect. Choose a GPS that can be used in different situations (in broad daylight, at night...).Opt for a model with a customizable interface (night mode, adjust brightness/contrast level of the screen...)
Sound and voice quality. This voice will accompany you during all your travels ...its better to have one you like. Some models allows you to download a voices from the internet (check the manufacturer's website).
The GPS mounts or cradles. This may seem incidental, but there is nothing more annoying than a GPS that "falls" when exposed to vibrations.
Autonomy, power and connectivity. Most GPS come with a cigarette lighter connector for use in a car. If you intend to use the GPS out of your vehicle, opt for a model with a good battery autonomy.
The quality of the information. Are the maps up-to-date ? Which area is being covered (country, Europe, worldwide)?
The ability to update maps. Today, most brands offer the possibility to update the maps. Some services require a subscription or purchase.
Radar and traffic info. Real time information about traffic are delivered using the RDS/TMC technology. Several options are available. RDS/TMC is either built in GPS or takes the form of an external antenna. Subscription to traffic information may be included (or not).
Points of interest. This is information related to gas stations, hotels, restaurants,....
Usability and Navigation
The ergonomics of the interface. Allows even a 6-7 years old child to map a route.
Processor. In the event that you have not complied with the given instructions, modern GPS can re-calculate your route in real time. The calculation time will of course depend on the processor.