I've been using a GPS unit for long distance travel since Dec 1999.
I cut my teeth using GPS when I got a Magellan GPS-315 for Christmas. The GPS-315 unit is a very basic, battery operated unit designed mainly for backpackers, boaters and such. You can get an auxiliary vehicle mount and 12 volt power chord, so you can run it indefinitely in a vehicle. This is a very crude system with no built-in road maps at all. It does have a database of Large, Medium and Small cities worldwide, but that is it. No roads or intersections whatsoever.
In order to plan a trip, you have to enter each way-point separately (Maximum of 30 way points per route an a maximum of 20 routes) and then you have to basically connect-the-dots between each way points to create your route.
One thing about this unit, by the time you are comfortable using it, you will thoroughly understand how a GPS works, the meaning of Lat/Lon coordinates, how to enter them into your system to create way-points, etc. One thing that I like about this unit is that you can set up a screen with fairly large text (Nice to have when your close distance vision gets harder after age 40) with Actual Speed, Heading, Bearing, time and Distance to the next way-point (as the crow flies), Current Position Coordinates, Elevation, proximity alarms (nice to program in known speed traps), etc. I still keep this unit in my truck because of this large text display.
The main thing I like about having a GPS unit, is that it helps reduce the stress of driving, especially when you are driving to an unfamiliar destination. I took many solo cross-country trips to unfamiliar destinations using only that crude Magellan GPS 315.
Four years ago I upgraded to a Garmin Street Pilot 2720 with all the bells and whistles in a modern GPS system, including remote control, an upgradeable complete US road and street map, a much larger screen, spoken directions with street name recognition and much more. The four things I don't like about this unit are (1) a fairly fragile power chord connector at the unit itself, (2) it has no internal battery, (3) the speaker is located in the power chord at the cigarette lighter end and (4) even though I have it set up for the "Quickest Route", occasionally it will take you where you want to go in a round-about way. Other than that is has been an excellent system, that will spoil you very quickly.
One thing to keep in mind when using any electronic navigation system is not to trust it blindly. You should alway have a good idea of where you want to go and what route you want to take, because some times you will have to override the directions you get. This will happen with any system.