What do you have, and why?
I have two Garmin units that I like very much. The first and oldest is a GPS 3 which I initially bought to use on my motorcycle. It works quite well, but the screen is a bit small to be checking out when you are at speed. However when you set your trip up in Delorme Street Atlas and download it into the GPS it gives turn arrows at selected distances before turns and which direction you need to head. Got to watch it on Interstates though as it indicates the ultimate direction, not the ramp. Once past that it's a piece of cake. Later I bought a Garmin Street Pilot. Large enough screen to see where you are going and again good usage with the DeLorme program. GPS is great, but the true power is in the computer programs that make them even more usefull. I haven't done it yet, but the ultimate will be to have a laptop with the program running and the GPS linked to the laptop. With that you get voice prompts and much more info (gas, lodging, service, hospitals and much more). If you are going to run the GPS outside of the car on it's internal batteries, you should keep clear of color screens as they absolutely chew up batteries at an alarming rate. The monotone screens are much better in this respect, but have a bit less Gee Whiz factor. Be sure to get a cigarette lighter plug at any case, and if you want truly amazing accurracy an external amplified antenna. Have got a fair ammount of experience with this if you want to go farther.