If you purchased at the right time (a year before the release) they "give" you half the cost of the phone for the phone but immediately apply it to what you owe, so basically they give you $0 as you have nothing to show for it.
I have no problem with this, as this is what you signed up for if you are in this program.
I think of it as more like a yearly lease or rent for a year program, with a rent to own option.
But you have to pay off the old one first and you have to meet someone shady on craigslist.