Currently sitting with my 6S - I was planning on upgrading this year like I do on Tock cycles, but the more I think about it, the less likely I feel like I will upgrade.
The two big possible changes that would make me reconsider are improved battery life (and it would have to be significant, especially considering I already have the battery case), and a better screen which, if rumors are true, only the more-expensive iP8/Edition/Pro/X will get. Maybe some cool stuff with NFC would make me think twice.
On my 6S, the processor is good enough for whatever task I throw at it. The camera takes decent pictures. TouchID works flawlessly, as does 3D Touch. I actually have a headphone jack which yes, I still use. Apple will have to really impress me come September to make me consider even jumping to a 7S, let alone an 8.