Hi Mike
I have a Navman. I have found it very useful on the whole, but you have to use it with a bit of intelligence. For example, after putting in my destination, before setting off, I always check the route it has chosen (there is an option to do this) because often there is more than one possible route, and a bit of previous knowledge and experience of certain roads, and traffic volumes, definitely helps. In fact in the UK I find it most useful for finding addresses in towns - often it will take me to the exact house - and I don't really need it for routes, although it is useful for monitoring how many miles to go, and estimated time of arrival.
The biggest problem I find is that the maps get out of date very quickly, and it costs