Buying in car satnav system.?

I am considering buying a satnav system for my car. (probably tomtom or navbloke ) does anybody know where i can try before i buy?
Answer
Hertz rents cars with this stuff in it. you can try there. also make sure that there is nit a monthly fee to use that thing. i have a magellan color gps. it works e where, not in the car.