Here's the most unequivocal statement I can make about the iPhone 6 Plus after long-term use: it makes the iPad Mini unnecessary.
Like many iPad owners, I was used to carrying around the Mini in addition to a phone. Out of habit, I continued to do so after I started using the 6 Plus. That didn't last long.
Within a few weeks I sidelined the Mini and, shortly after that, gave it to Verizon via its recycling program. It wasn't a difficult decision.
With a 5.5-inch screen, Apple did a good job of finding the phablet sweet spot. With just enough screen real estate to make it usable as a tablet (landscape view helps) but not too big to make it impractical as a pocketable phone.
To wit, it's decidedly bigger than the second-generation 5.2-inch Moto X (which looks surprisingly small sitting next to the 6 Plus) but doesn't feel like an oversized phone, as 6-inchers can. (That said, who knows, maybe in a year vendors will decide a 6-inch display is the minimum for a flagship phone.)
What else is there to like about a 5.5-inch Apple tablet after using it for more than a month? You can actually snap a photo with one hand. The first tablet-class device I've carried that allows you to do this with relative ease (and without making you look stupid).
The bigger screen also means it's easier to review photos. I could never do this satisfactorily with smaller phones. And the 6 Plus takes decidedly better photos than the gen 2 Moto X (a great phone otherwise).
There are lots of other spec-specific reasons I won't address here (e.g., performance, display quality, and weight distribution), as they have already been covered ad nauseam by reviewers. Suffice to say, it functions as a small tablet and that's good enough.
The Mini's heyday came and went quickly. And the 6 Plus just adds an exclamation point.
