I've owned several GPS/Nav systems. I've used a laptop with a gps attached to it (Street Atlas), a Palm PDA with the Tom Tom navigation program data and gps receiver (the same database you get with the Tom Tom stand alone systems), and my current one is the Magellan RoadMate 800 with the built in gps receiver.
Here is my experience. They are only as good as the data that they use. Most of them use data from two primary providers, but it must be up to date, which means the device must be able to upgrade to new maps or it will quickly become obsolete. I found the laptop, with Street Atlas, the easiest to set up routes, especially if you want to avoid some roads, or want to choose your preferred route over the computed route. It also allows you zoom in and out and see the whole route, or a segment in high detail. I had a mount that I put the laptop on, and it sat on the center console. It's a little bulky, but it works.
The Palm PDA with Tom Tom installed, was the next best one. It's a little hard to see, when mounted on the dash, but it works. The only problem is that even though it was supposed to allow you to select "vias" (other routes rather then the computer route) it didn't always keep to the new route. It wanted to route me back over to the road I was trying to avoid. Also, when I used it to get me to a relatives house, out in the country, it tried to route me over a logging road, as that was supposedly the shortest route. Not good, if you are towing a trailer.
I tried it again with a Magellan RoadMate 800 and that was a mistake. It is very hard to use (i. e. doesn't like you to chose roads), only let you program one leg at a time, and if you went off route (i. e. tried to take a road you knew was better or quicker) it keep wanting you to turn around and go back to the old route for miles beyond the change.
Here is my advice. Most of these units are very good at getting you to a destination in town (i. e. restaurants, etc. ) but I'd be very suspect using one while towing in out of the way places. I've had every one of them try to take me down a bad road, dead end, or wrong way on a one way street. Not good when you are towing a 30'+ trailer. They also work pretty well while traveling down main roads or freeways. When I really have to go to an area I don't know much about, I look at a good map, so I know where I really should be going...
