My brother has a GPS plug-in for his laptop. We used it for a trip a few months back and even with him navigating, it was a MAJOR PITA. No touch-screen, slow satellite acquisition, slow to re-route directions, difficult to zoom in and out, etc. I can't imagine trying to use one while driving alone. Where would you even set it, loose on the passenger seat?
I've been using a Lowrance 500c for a couple years. Bought it for its large 5 1/2 inch screen. The combination of hard buttons and touch-screen commands is, by far, the easiest to use GPS unit I've tried. Very intuitive.
http://reviews.cnet.com/car-gps-navigation/lowrance-iway-500c/4505-3430_7-31411013.html?tag=pdtl-img