TidBITS editor Glenn Fleishman spent weeks driving aimlessly for Macworld, where his overall look at GPS navigation for the iPhone appears, along with reviews of nine apps and the TomTom car kit. follow link
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Head-to-Head Tests of Nine GPS Navigation iPhone Apps
Aren't windshield mounts illegal in some states, including California?
Follow the link to the article, which explains that. (Short answer: Maryland has some limits; California just amended a law to allow certain windshield mounting locations.)
More the problem with windshield mounts in some cities is that they mark your car as potentially having a GPS in the glove compartment, ready for stealing through a broken window. Been there, done that (in Brooklyn).