Since I've now got hold a 6 plus, I have some thoughts of my own at this stage.
I happened to leave my iPad mini retina at work over the weekend. That forced me to use the plus for the entire weekend. I've found that I can make do, but there were times I was missing my iPad, eg gaming, watching and reading.
Sure, I could do all those things on the plus, but I always remembered that they were far better on the iPad. Maybe the plus' advantage is that I might be able to do all those things on situations with the plus on the go where previously I wouldn't have bothered bringing the iPad or connecting it to my phone's hotspot to do something.
So far, I can see why Apple left out a plus size phone until now. It's really a compromise between two. When you use it there are times you miss the larger iPad. At the same time, when on the go and mobile, there are times when you miss the smaller iPhone.
The only chief area it doesn't compromise at the moment for me is battery life. I'll have to see whether it's enough to keep me from going back to the 6.