It is useful but be careful you don't follow the electronic maps religiously as sometimes you'll end up in somebodies yard or worse.
That's why the voice navigation feature had been implemented. You eyes are concentrated on the surroundings. Your extremities - on driving. And you ears listen to the prompt directions...
I was entirely convinced of the voice navigation supremacy when once missed my exit in the unfamiliar remote place in the dark rainy night and completely lost on a complicated freeway ramps system. It was not possible to see any road signs behind the rain, and dense but speedy traffic was still there. So I just turned on (in 2 touches of the screen) the voice on my PDA (I have it connected to the wireless GPS unit and running TomTom software) and was at the door of my friends in no time without any stress feeling. Actually I felt like piloting a plane in the fog using only the cockpit gauges (instrument flying). My only desire was for some short range circular radar to see the other cars better