I am currently using a Nokia N97. I assume the Nokia navigation on N8 would be similar to N97.
The navigation on N97 is not bad. You can download (official) voice guide (for free). After you got the voice guide, it is similar to a stand alone GPS unit, like Garmin and Tomtom.
Compare with a stand alone GPS, the disadvantage is it relies on data. It costs your data allowance no navigation (but the cost is not too high, ususally under 200KB each trip). The advantage is you can always get free update for the map. You always get latest official map for free.
The GPS in the N8 is very good indeed and is free. You do not need an Internet connection to navigate and you can download maps for about 180 countries, all for free. As well as having an excellent camera it also has an FM transmitter so that you can play the songs stored on you phone through your car stereo or home system. How good is that?