Driving in The US - sat nav recommendations?

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Shortly going to be driving along the west coast for around 2 weeks, covering approximately 2000 miles so thinking a sat nav may come in handy at some point.

Does anyone have any recommendations for one to use? Don't want to pay extortionate rates to hire one so was thinking or purchasing either a garmin or tomtom so I can use it at home too.

Anyone used the iPhone garmin or tomtom apps for the USA?

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Thanks for all your suggestions!

I've downloaded Nav Free USA, and have purchased a windscreen mount for the iPhone - if I find this doesn't do the job when I get there, I'll nip into Best Buy or Wal Mart and buy a Tom Tom!