I have a VDO MS5000 installed in my Scenic which is a hard wired unit and can highly recommend. As it takes a feed from the car speed pulse its spot on for acuracy as it knows where you are even if the sat signal drops out as it also has a gyroscope inside which it uses along with the digital map on CD to know exactly where you are.
I also have a Tomtom One which I was given which I took to Florida earlier this month which was brilliant, with the Tomtom you can even edit your own POI's. Before I went i used a site that used google maps to let you exactly pinpoint where you want to go then click and save the co-ordinates into a POI file. I made a custom POI file for the places where we wanted to go and it too was spot on.
Hi there, I have a system which I take from car to van, etc. Its installed on my Sony Ericsson phone and supplied by Route 66. I find it invaluable for my business, as it allows calls to be made and received whilst giving directions.
The main advantage is that it cuts down on yet another item to need a power socket.
If you have a compatible mobile (they supply for Nokia also), then its another option for you to consider.