The Sprint coverage map is VERY misleading. When you first look at it , there is a LOT of deep purple across the USA. But if you drill down just one or two clicks to zoom into it... the coverage quickly disappears as most of it is on roaming.
If you are thinking of getting a hotspot (mifi) device from Sprint for use during traveling... just be aware that they will not work on roaming. the most you will be able to use it for is 100mb-300mb before you get nothing.
ATT and Verizon are the only way to go - for using internet on the road.
Kathy