My wife would like to upgrade from her 6 to an X. Since she tends to keep her phones a long time and never is interested in upgrading early, I'm planning on buying an X outright for her.
I, on the other hand, am nearly always interested in new features, so I upgrade often. I've been using the IUP for a couple years now and it's been working well for me. It means an endless payment program, but in return I get a zero interest loan, AppleCare coverage, no hassle selling an old phone, and get a new phone often.
As best I can tell, if you buy a phone outright, and get AppleCare, the costs ends up about the same as using the IUP. However, lots depends on what you can recoup on the sale of the old phone. Right now, since the 8 has been released, what I could get for my 7+ is far less than what it would take to pay it off. Makes me think I'm a bit ahead since I plan on upgrading.
Some people mention they can get better prices by selling the old phone before the new one is released. True enough, but I'm not sure what they do for a phone in the meantime.