Les, a suggestion: there are companies in UK that hire satnavs on a daily basis. The model you're interested in may not be one that they use but what they have may give you a base for comparison.

I have a nav program in a PDA which uses an external bluetooth GPS. It's probably not as user friendly as the dedicated satnav systems but I wouldn't go to any unfamiliar area without it, and even on trips that I drive almost daily it's nice to have the speed and ETA read-outs. The voice may be turned off if it annoys.

Rowan - agreed, they don't always plan the best route. The aviation industry is probably the greatest user of electronic navigation systems. Their equipment is generically called "navigation aids". That "aid" means they are just that. It makes sense when venturing away from home to do a bit of research on a good map or street directory and, if need be, plot waypoints into the GPS to force your preferred route to be used.