Do the navigation GPS units come with handlebar mounts so that you can see the map while riding? Or maybe hear directions instead (problematic with wind in your ears)? Or is it used mainly at stops? I'm contemplating at Edge 305 because I like the idea of storing rides on the web along with weather, elevation, etc. to go with performance data. The breadcrumb trick is good if you are repeating a ride or if you have downloaed someone else's ride. And there are a lot of those at motionbased.com.
A GPS system that did ALL of the above plus an ear piece for verbal instructions would be cool (and expensive)