I've cached the whole city I live in there and use it daily. I also have GPRS in the car so I can do routing no problem, but that's not very common for me.
There's a cache builder application that was made by a user here at mp3car.com:
Google Earth Cache Builder
That's what I used... however, I did go over most "common" places I know to make sure they were cached. 1 City took me about 300Mb so caching an entire state would be kind of crazy.