My experience:
-I entered the IUP program 10/16/2015
-March 2016, I called AT&T to verify my iPhone was unlocked as I was travelling to Europe. AT&T said it was not, give them 72 hours to unlock it and I would receive an email confirmation. Never got it, called AT&T, had to send them the receipt that I bought the phone with the IUP, then they unlocked the phone.
-Sept 9, 0301?, shortly after 3AM EST, no stock for reservation for iPhone 7+ in my state and three surrounding states within reasonable drivng distance. Got message that if I couldn't find the model I was looking for, "Check again on Sept 17", wtf.
-checked every morning and every night for the next several days, noticed stock varied but never what I wanted (128GB, silver or gold)
-Sept 13, called 1800-My-Apple. Was told I should call back on the day of release at 8AM to reserve a phone, that "all system issues would be fixed by then"
-Sept 13, called again, waited an hour and gave up
-Sept 13, 11:30PM, noticed all stores suddenly had stock in almost all models, capacities, and carriers! Reserved iPhone 7+ 128gb gold for pick up on Friday at 5:30PM
-Immediately noticed my reservation wasn't showing on my Apple Account nor Apple Store app on my iPhone. But I have an email confirming it
-Sept 14, called Apple, three times. All three specialists have no record of my reservation in their system. All three could not fix issue. All three recommended "just showing up" at the beginning of my reservation window.
-Sept 14, noticed a thread on this forum saying another user was told you need to pay 12 installments prior to coming in to pick up. Called Apple, again, was told I could pay my 12th payment in store
At this point, just crossing fingers that I am not driving an hour to an apple store tomorrow to pick up an iPhone I might not be able to get. I'll stick to IUP though, I don't want to use the AT&T Next program because I have a grandfathered unlimited data plan