Have a couple of Garmins which have been great but people also rate Tom Tom. Stick to these brands and decide what features you want (speech direction, Euro maps etc) and do a price search . I also like a big screen.
Have MMI Navigation system plus very good now with 7 Digit Post Code and Google maps.
Went in a friends Car and I used Google Maps on my Phone took us strait there with speech directions and its Free.
Save my self some money next time I order a Car.