One of the more irritating shortcomings that I have found on the N8 is that the Navigation application will not route its audio to my Blue Tooth headset when it is activated. The audio comes out of the phone's speaker. I like the audio to come through the headset because it is easier to hear the commands and because the audio will not bother the other occupants of the car.
I have not tested the navigation extensively, however, I have used it to get home from my errands around San Francisco. Generally, the routes that it chooses do not match the route that I would take. In some cases, its routes seem rather bizarre. I follow my route and have found that the N8 recalculates routes quickly.