I've been with sprint somewhere between 8 and 10 years, and in my experience pretty much everything grand they have promised in the future has turned out to be a dud or otherwise a gimmick.
They were one of the first carriers with 3G, and their EVDO was supposed to be so fast, only it wasn't. And then as EVDO and CDMA2000 in general advanced, they haven't even bothered to upgrade it to keep up with other carriers. For example they're still on classic EVDO, whereas Verizon has long since upgraded to EVDO Rev A which not only does higher data rates but also supports simultaneous voice/data.
Then they went on this long advertising campaign about how they were going to be the first carrier with 4G. While in one sense it was true, they were the first carrier to deploy WiMAX, which is technically a 4G standard, their implementation of it was just downright sh*tty. For starters, the band they picked gets horrible reception indoors. And also, the speeds they offer don't even meet the FCC definition of what 4G speeds should be.
Let's not even get into the fact that in spite of being first, Sprint still has the smallest 4G network.
And then I remember for a long time after the Evo 4G came out, there hadn't been any decent phones since for a good 8 months (didn't bother me too much, because I was happy with my Evo) however after a long time of nothing, they made this big announcement of a new "industry first." Gee whiz, it turns out that this "industry first" is nothing more than a two screen android phone, called the kyocera echo. Well hold me back, the "industry first" two screen phone. Lame.
I've only stayed with them for so long because their services are so cheap.