Surely from the 5/5s/SE you want a 7...but from the 6 and especially 6s it seems like a waaaaay overly incremental upgrade
It depends on the use case, in my opinion. I take a ton of pictures and videos with my iPhone and the camera on the 7 Plus is way better than anything in the 6/6s series, especially in low light and when zooming. Also, portrait mode is really, really cool. That being said, I'm generally an annual upgrader anyway and don't nearly represent the general population of smartphone users.