I've owned the 6 Plus for nearly a year and sold it last week. I'm going to get... a regular 6S!
The 6 Plus' display is amazing and battery life is exactly what I hoped it to be (excellent). OIS and horizontal home screen are not worth it, though. So, why the switch? A few things:
1. 6 Plus is just a bit too large (a bit too long, a bit too wide)
2.6 Plus multitasking performance was not good enough
Regarding point 1: it's too large as soon as you sit down while having the phone in your pocket and, especially, when you are doing something 'active' (running, cycling, etc). For iPhone 7, it might be more workable if they manage to reduce both the width and height of the device.
Regarding point 2: the 6 Plus (on iOS 8) actually performered worse than my iPhone 5 (on iOS 7). It had the occasional stutter up which I believe has a lot to do with the downsampling of the resolution. Also, Safari would force shut itself as soon as you visited a page with lots of images and/or videos. Never happened on the iPhone 5.
Secondly, (multitasking) performance was terrible. Usually I'd be able to keep one app 'ready to go' in the background. If I had some bad luck, it would be flushed out of RAM immediately. If I had some good luck, it would keep two apps 'ready to go'. On my older iPhone 5, I was able to open up background app Nr 5, Nr 6 etc and I would be able to continue rights where I left off.
With the 6S Plus, I believe that multitasking performance will have been improved because the amount of RAM has doubled. Not sure if the occasional stutters are gone, since it's still downsampling all the time.
Conclusion
So, for me: I'm going from a 6 Plus to 6S. Looking forward to better performance and also, especially, the higher mobility.
If you're not sure whether to go for the 6S Plus or the regular 6S: follow your instinct. A year ago, I definitely wanted the bigger display and better battery life, even though I already had some worries about its size. I would have regretted it if I hadn't tried the 6 Plus.