I'm not from the US so I don't know quite what the pricing is like over the pond, but here in the UK the iPhone subsidies are looking less and less enticing. When I signed up for a contract with the iPhone 5, getting a subsidised phone saved me money compared to buying it outright and signing up for a rolling monthly tariff.
I did the same calculations for the 5S, and it's substantially cheaper to just suck it up and buy it outright. Plus, you then have the added bonus of an unlocked device that isn't tied down to a carrier. If the networks think I'm going to be giving them £50 every month for the next two years they can jog on.
I partially did this on O2 just a few days ago - I used the new split plan to get a 5S, which I paid for partially up front (about half the cost, but you can choose how much from minimal up front to fully buying the phone) plus the airtime contract.
I think it's great. The O2 guy also mentioned that I can pay off the phone part of the contract at any time to reduce my monthly bill with no early payment penalty etc. He mentioned that at that point they would also unlock the handset so I could travel with it (I visit the US frequently).
Given that I paid for half of the phone up front what they've effectively done is given me a no interest loan for the the remainder since I'll probably chip it off over the next couple of months and then own the thing outright, and just carry on with the cheaper airtime contract (which is £3/m less than the iPhone Simplicity plan I was on with my iPhone 4 before this).
TL
Before looking at these plans I was just going to buy the phone from Apple outright.