I went from the 6 to the 6S Plus. Gave it a fair chance over the last week, but I'm returning and switching to the 6S. For anyone else curious about the Plus, I recommend doing the same, because your experience might be different.
Sure, there were moments that I liked the Plus a bit more than the smaller screen, but at no point did I say to myself, "Wow, this is way better than the smaller iPhone." Being able to comfortably pocket my phone, and operate with one hand — for me, those are more important attributes than extra screen size and battery life. While OIS would be great, for any serious photos I'm using a much better Canon DSLR anyway.
That being said, I would probably switch to a larger display in the future, if the form factor is a bit smaller than the current Plus.