I've finally thrown in the towel and given up on my iPhone 6s Plus -- I've come to realize that the compromises really aren't worth it. I had switched to the 6 Plus from the 6 within a week of launch last year, I loved it. Some people thought I was crazy, others knew and understood why it was the superior device. Now I no longer feel that it is.
After going back and forth between my 6s and my 6s Plus, one thing I couldn't live with was the added heft that the 6s Plus carries. It's just no longer comfortable to use, and everyone comments on it. I understand that they added display components and thickened the aluminum, but it's a trade off that I'm no longer willing to live with.
As someone that lives on their phone, I need it to feel like a phone. I dealt with it before for the added battery life, but with iOS 9 and it's battery saving options on top of it's optimizations, there's no longer as grave of a difference as there was before. I've accepted that I no longer have OIS, but I also only take photos of my dog and food. Neither of which are very important for pro photography.
As noted before, battery differences aren't as large as they were before, due to the new optimizations. But also, using the iPad charging block, the 6s actually charges relatively quick when you're trying to top up. I can plug it in when I get in the shower mid-day and I've gained 20% by the time I'm done in the bathroom, this is sufficient for me.
If I'm in the car, I'm always plugged in for music. These brief top ups are enough to get me through the day -- I've learned that I use my phone as a phone, I make calls and I send messages; nothing more, nothing less, therefore I'd prefer it to feel like a phone in the process. The added comfort of something 'normal' has been a relief.
For those considering it a downgrade, I'd think twice and give the standard 6s a try before hanging it out to dry.