4" to 4.7" is much easier to swallow than the jump to 5.5".
I was a very happy Plus owner until I got into some real-world situations where I was trying to use my phone like I did my 5s, sending quick texts, getting my music ready for my car, and taking pictures, and well... it was pretty difficult to manage. It all became a much bigger deal to do any of this stuff that I consider phone essentials. I returned it for a 6 and I just enjoy it a lot more. The screen size bump is nice and the usability is pretty much the same as the last generation.
I mean, the Plus is an awesome device and all, and the screen and battery are unparalleled, but I have to tell you, the size becomes a burden if you're happy with using your phone how you are now.
If you went into the 6 announcement without a big emphasis on the size argument then I see no logical reason to why you'd want to jump into a 5.5" phone, which is a little ridiculous unless you know that's it exactly what you want.
The real issue isn't the screen size but the height of the device. They have to trim the bezels down for this to become a practical device.
I went into the 6 announcement totally thinking that I'd just get the 6, but after reading/watching reviews, there is a part of me that wants to give the Plus a try because of what I primarily use my phone for which is video content, web browsing and social media. That aspect I'm 100% sure of.