Like several others in this thread, I had been experiencing 6+ envy for a little while now. I got the 6 on launch day and have enjoyed it, but several people I work with have gotten the 6+ and seem to really love the big screen. Finally, I decided to give the 6+ a go and bought an unlocked version a few days ago, figuring I could return it within the 14 days if I didn't like it. Here are my thoughts for anyone who cares:
Screen: While the screen is definitely, noticeably nicer than on the 6, it's not leaps and bounds better. I always thought Apple was low-balling the "Retina" threshold at 326 ppi by just a tiny bit. At this resolution, I notice the occasional jaggie, though it is rare and I have to be holding the phone close to my face (lying down in bed is the most common instance). With the 401 ppi on the the 6+, I didn't notice any jaggies, which was nice. But, beyond that, I didn't really notice any huge differences between the screens beyond the extra space on the plus. Colors were the same, and I even found the screen on my 6 to be a bit brighter than on the 6+. I don't know, I guess with the way some of the 6+ owners on here describe their screens I was expecting more of a dramatic difference. The screen on the 6+ is definitely better, but it didn't blow me away.
Typing: Typing in portrait was definitely improved on the 6+ using two hands thanks to the wider physical dimensions. I finally felt this was a phone I could comfortably hold and fit two thumbs for typing. Typing in landscape was another story though. With all the focus on landscape mode on the 6+, I was expecting the typing experience to be great. It wasn't. Rather than make the keyboard any bigger, Apple just added 2 more columns of function keys from the 6 and move the letter keys further to the middle. I was shocked to discover I now had to stretch my thumbs to reach the center of the keyboard thanks to the massive bezels on the top and bottom of the 6+. Apple needs to redesign this keyboard or offer a split keyboard option like on the iPad. Typing one-handed was a no-go in either orientation. But, I was expecting that going in so no surprise there.
Camera: The OIS definitely helps, but only subtly. I compared a bunch of photos taken with both phones and the 6+ does better with small, fine details, especially in low light. Beyond that, no real difference between the two.
Battery Life: Definitely better than on the 6. But, I never really had a huge problem with getting through the day on my 6. Occasionally, I need to throw it on the charger for 20-30 mins if I'm going out in the evening, but that's not a huge hardship.
Ergonomics: Terrible for me. I really, really wanted to love this phone for the bigger screen, and better camera. But, I just could not find a way to hold the thing in a way that was comfortable to me. In portrait, I felt like I had to hold it at least half way up or the weight of the top might cause it to flip out of my hand. But, doing this made reaching the home button a stretch. Reaching the power button required at least a little bit of sliding, depending on where my hand was. In landscape, I already mentioned the issue with typing. But, just holding the phone one handed in landscape felt a bit awkward too. Surprisingly to me, pocketability really wasn't an issue. The 6+ fit into every pair of jeans and shorts I have without much issue. I did notice it there more than the 6, but it was never uncomfortable.
Verdict: I'll be returning the 6+ tomorrow and sticking with my 6. The bigger screen is nice, though it definitely doesn't get anywhere near iPad territory in terms of real-estate (i.e. it still "feels" like a phone much more than a tablet). The camera and battery life are better, though the differences are not as dramatic as one might expect. At the end of the day, the ergonomics were what killed it for me. I just couldn't find a comfortable way to use the thing. The screen on the 6 may be smaller, but it doesn't feel cramped. And, I find the tradeoff to be worth it for a phone that is vastly more comfortable to use (for me).
In the end, it's a personal decision and there is no wrong answer. I'm glad I had a few days to try out the 6+. It's a neat device and there is clearly a demand for it. But, for me at least, smaller is better.