Size and viewability of display: GPS screen should be large and glare-free (optional sun visors are available check with manufacturer), and show basic information such as two or three-dimensional map view, current time or arrival time, distance to the next turn, current street name, and distance to destination, current speed (some units have the current speed limit as well).

Data entry: Check how easy it is to enter addresses and routing preferences. Is the typing pad user friendly, does it have voice activation, is it bluetooth enabled, remote control etc..

Installation and portability: How easy is it to setup and remove ( they are targets for thieves). Does it have a case or screen protector?

Features:
Most manufacturers offer these features but you should confirm.
Battery for use away from the car for a short amount of time (no more than a couple of hours).
SD memory card slot.
Option to download map data for other countries.
USB PC connection for GPS unit.
Storage of your "Home" address.
Traffic updates.
Advanced Navigation: Calculate fastest or shortest route, exclude highways and toll roads, alternate route to avoid traffic.
Display car speed, speed limit, distance to destination, and estimated time of arrival.
Display street name and distance to the next turn.
Option of two- or three-dimensional map display.
Volume control for voice instructions.
Points of interest (POI): schools, police stations, car parks, fast-food outlets, post offices, petrol stations, airports, railway stations, hospitals. Option to add your own points of interest.
Advanced lane guidance tells you when to move to the exit lane and displays complex multi-lane manoeuvres clearly.
Languages and gender choice; You can select a number of other languages for the display and voice instructions (such as French, Spanish, German, Italian, Swedish, Danish and Dutch) as well as accents.

General Recreational Vehicle GPS Information

Garmin United States Home
Delivering innovative GPS technology across diverse markets, including aviation, marine, fitness, outdoor recreation, tracking and mobile apps.

TomTom, portable GPS car navigation systems
GPS satellite navigation solutions for mobile phones, PDAs and motor vehicles.

Magellan GPS - Car, Handheld, and Mobile Navigation & Accessories
Magellan delivers innovative GPS technology for vehicle navigation, outdoor recreation, iphones, and GIS.

POI Factory
This is where GPS users get together to share locations and discussions with other GPS users.

The Global Positioning Systems (GPS) Resource Library
This is a page of links containing information about the Global Positioning System (GPS) and handheld GPS units.