I use a car sat nav or my iphone with a waterproof case and stem mount - got the stem mount and case from Ebay -
http://www.ebay.co.uk/itm/Motorcycle...item231e016aca
The car sat nav works but if I was using it regularly I'd definitely get a proper bike sat nav, it has its limitations.
I don't really find the car sat nav much different to the free maps on my iphone which I also sometimes use.
Alot depends on how much you want to spend and how often you are going to use it.