I really like T-Mobile's program since a lot of my data usage was disappearing into streaming music (namely Google Music/Play Music). However I will fully admit it is anti-competitive since if you're a new music streaming service just trying to make it, you won't get onto T-Mobile's free list (and even Google Music/Play Music wasn't until extremely recently, like even two months ago it wasn't).
However as much as I like and benefit from this program, I think the world would be a better place if all data was treated equally. I'd actually be willing to sacrifice this program if it meant a more fair internet, and I benefit from it...