I carried a 6" phone for almost 2 weeks. It was a Nokia 1520, the largest Windows Phone currently available, running a beta of Windows Phone 8.1.
It didn’t suck because of the software. The Windows Phone OS is actually wonderful, and although there’s a dearth of applications, the built in suite of apps works quite well, and look great to boot.
It also didn’t suck because of the hardware itself. The Nokia 1520 is very fast, has an amazing camera that shoots RAW, and is thin and comfortable to hold.1 And that screen, my lord. Web browsing felt more like using an iPad than using a phone – that is to say, it felt wonderful. Likewise for watching any sort of video, or viewing photos.
It sucked because it was just too big to carry. It didn’t fit in the front pocket of my jeans, and it didn’t fit in the back pocket of my jeans. (Even if it would have, I’d be wary of sitting on it, and having it flex so much that the screen might break.) It didn’t fit in any pocket that was designed for “cell phone use”, and it didn’t fit in any of my friends’ pockets. My friends mocked me. My family was confused.
I’ll probably still buy a 5.5" iPhone, though.
If the rumours are true and the “iPhone 6 Plus” has a better camera, and/or a larger storage capacity, then I will put up with the inconvenience of carrying it, because all the other details of carrying a huge phone make it worthwhile.