I drove from Naples to Mpls a couple of days ago.  The only significant
areas where I didn't have coverage were:

1.  Along US 41 from Clarksville TN to I-74 near Danville, IL.  There was
coverage in some of the towns along the route (Terre-Haute, and Evansville,
and a couple of more), but there was no coverage in the rural areas.

2.  Along I-90 / 94 from Rockville to Eau Clare WI.  Coverage was spotty,
except for Madison, and some minor coverage in Wisconsin Dells, and Tomah.

Overall, pretty amazing compared to what coverage was like a couple of years
ago.

Also:  Used CSD data connection to connect to my Mpls ISP to get my e-mails
for much of the trip.  Worked flawlessly (but slowly - 9600bps), even while
roaming on Einstein along I-90.