T-Mobile rolled out Data Stash today, which lets you roll your data over month-to-month, and just sounds super awesome. It is, too, at least at first blush. There’s not much trickery, here, but the wizard of data (I literally just made that up) John Legere is now firmly staking claim that data is important, and we want more of it. Is Data Stash all its cracked up to be, though? We examine the nuances of T-Mobile’s killer concept so you know exactly what you’re getting yourself into -- if you're even eligible.