Which add on navigation brand is best?

good choice, i just bought my wife the 660 for her birthday. mounted low on the windshield for now but i like the vent clip idea, where did you get the clip from?
the 660 worked well on a 5 hour trip i just got back from. the linking to the cell phone via blue tooth is really cool, the cell phone contacts #s appear on the nuvi and you can touch dial #'s using the nuvi screen.

the drawbacks i see are it missed alerting us to a 1 hour traffic jam due to construction (it does have a feature that it will reroute you around delays if you choose)_. as well the volume could stand to be louder even with it at 100% you have to listen carefully to hear the directions esp if you have the radio on. have not updated the unit by hooking it up to the home computer yet, maybe that will help with the traffic updates? otherwise a great unit i hope you enjoy.