John Gruber, at the end of his iPhone 6S review:
New-number iPhones (4, 5, 6) are about showing off Apple’s design prowess. The S models are about showing off Apple’s engineering prowess. Storage capacities and battery life are unchanged from last year’s iPhones. Everything else — the materials it’s made from, the performance of its custom CPU/GPU, the quality of the cameras, the smoothness of the user interface — is noticeably, tangibly improved.
I like this way of framing Apple’s annual phone releases. If you buy every other year like a normal human being, I think the S cycle is probably the one to be on at this point.