Although in a strict financial sense, the interest free loan available using the IUP makes good financial sense, psychologically, the program is a money pit. Everyone who joins it upgrades to a new phone each year, perpetuating monthly iPhone payments until they die. Most people not on the program don't upgrade their iPhone every year so they're buying fewer phones over the span of 5 years (for example) than someone on the IUP.
The IUP only defers the first year of payment (in a sense) once you get sucked into the system, but the illusion of saving money is only that. Eventually when a person comes to their senses and realizes that this program sucks them into buying more & more iPhones they'll see how they got suckered in to forking out cash to Apple every year.
When they finally grow weary of the perpetual payments year after year, they'll still have to pay off their last phone when they exit. In the end... all they'll have to show for it is having purchased way too many iPhones over the years, all at full price and essentially giving away all their perfectly good year old iPhones back to Apple for half price.
Sounds like a horrible deal to me.
