i’ve recently downloaded a free iPhone app called Skobbler.
its a sat nav like all the others but i have used it a couple of times using just the navigation voice to guide me (as i dont have a cradle on my bike) and its got me to where i needed to go a few times without hassle. this has been in/around the city.
the maps are OpenStreetMap created so basically people have contributed them - this may mean it doesnt have accurate maps for smaller towns… have a read about it: http://www.skobbler.co.uk/
if you’re the type that listens to music through earphones, the navigation voice directs you without interruption to the music!
not really sure how it compares to other brands, but i thought it was decent.