I have the Simply Everything Family Plan. On it, you get unlimited roaming data (though they make it look like you don't on their website). They intentionally don't advertise it, because they do not want to encourage people who live out of their coverage area to sign up. These people are called permanent roamers.
However, there is some fine print. If you consistently use more data off their network than on the Sprint network than they may consider you permanently roaming. I had this problem with AT&T, before moving to Sprint. But it was because I moved to an out of service area. AT&T let me out of my contract.
Since moving to Sprint, I keep watch of my usage to make sure that I am not in danger of being lopsided in my usage. So far, my Sprint Data to Roaming Data ratio runs about 3:1. If it ever started approaching 1:1, I'd change my roaming habits.
On another message board, a user pointed out that in Sprint's Terms of Service, it lists that there is a 350mb cap to roaming data. However, I have not had a problem in the past exceeding 350mb. I think it might be limited on certain plans at 350mb.
Also, I may be grandfathered in with unlimited roaming data. Be sure to ask a lot of questions if you are starting new service and confirm everything. The unlimited roaming data may not exist anymore for new customers.