I guess the question was for those who do have the in-car navigation system in their car but I would find a laptop or even a Pocket PC clumsy in the car. Instead you can often use your own phone with amazingly good performance. A bluetooth GPS device with less than 100$ plus the navigation application like Route 66, Wayfinder, Navicore Nav4all, etc. etc. would be sufficient.
So far I only have experience of an inbuild navigation system from test car drives, I leave comments on that to those who have one in their own car (have one in my "in-order" car but mainly because it is standard, Command APS in a similar car where it is not standard would cost me some 3500 euros, not an obvious choice)