Seeking the best sat nav for use in London, as my job involves delivering prestige vehicles to clients in all areas of the capital and i've found the Garmin 660 lets me down sometimes as it fails to find some of the smaller streets.
In my experience Sat Navs do work in London, but they really aren't the ideal and no substitute for knowing the area. I rarely go the way the Sat Nav indicates when I am in areas that I know, it's very good when I have no clue though.
