I have three phones on my account, 2 being iPhones, on a 2 GB shared plan. When the iPhone 6 came out, I looked at NEXT to upgrade, even though I had a year to go on my contract. I relied on this statement from the Nov. 11, 2014 ATT NEXT promo:
"Upgrade: If upgrading to a new phone the discount is only available via AT&T NextSM or when your bring your own phone or buy an new phone at full price."
Since I was essentially purchasing a new phone at full price (from AT&T), I signed up, expecting a $15/month discount on my plan. At the next billing cycle, the increase for the new phone purchase was on the bill but the discount was not! After speaking to a service rep and a manager, I was informed that the discount didn't apply to me. It turns out existing customers who had a shared data plan discount prior to the NEXT program signup were not "eligible" for the NEXT discount!
So in essence, if you are an existing contract customer with a shared data plan, you get no benefit from the NEXT program but you do get the "privilege" of paying AT&T (on time) the full price for a iPhone locked to AT&T. (Of course, you could spend the same amount by purchasing an unlocked phone with no AT&T tie-in.)
A more honest promotion of the Next program to existing customers would be to notify them in the NEXT promotional literature that they are ineligible for additional service discounts if they sign up for NEXT.
I consider this bait and switch an abrogation of the long-term relationship I've had for over 10 years with AT&T wireless. Once my contractual obligation is over on the other two phones, I intend to pay off the Next phone and demand that all phones be unlocked so that I can go to the competiton!