Some of the gps apps actually use maps that you download essentially making your mobile device a true dashboard GPS and not just a phone that uses data for the maps. This would work good with a iPad or another tablet device. I think Navigon is one option to download a full map. Just a word of caution...those maps are huge!!
I personally use a Garmin And love have a separate device as my GPS since it's so much easier to see.
I use the phone to locate local things like the above posters.
Thanks!
Jeremiah