The J.D. Power and Associates just released the 2008 Portable Navigation Device customer satisfaction rankings with Garmin taking the top seat and TomTom following closely behind. These scores are a composite of six factors that include: ease of use, routing, system appearance, speed of system, voice direction, and navigation display screen. 

Garmin ranks highest among portable navigation device manufacturers, performing particularly well in the display screen, ease of use and appearance factors

TomTom follows Garmin in the ranking, performing well in the routing, speed of system and voice direction factors

Personally, I have never been a big fan of Garmin interfaces; who am I though but a lowly gadget blogger.

JDPower via GPSTracklog