I upgraded from the 5 to the 6, largely because I felt my 5 was on its last legs...the power button was wonky, battery no longer held much of a charge (no, it didn't qualify for the replacement program), and the Lightning port was behaving oddly; frequently I'd have to insert it 5 or 6 times before the phone would register as charging (yes, I cleaned the hell out of it). I'd had it for a full 2 years, so I felt I was due for an upgrade anyway.
So given all of these things, plus the larger screen/noticeably superior camera/Touch ID etcetera, it definitely feels like a big step up. I've only had it for 3 days, so jury's still out on the OS and battery life, but I don't regret my decision one bit.
All that said, were my 5 in pristine condition and behaving as such, I may well have stuck with it. It still feels fairly snappy (certainly compared to my wife's 4) and is absolutely a solid phone. Of course tech is constantly advancing, so waiting will always yield a better/faster product, but eventually one must, uh, poop or get off the pot, so to speak. For me, that time was now. YMMV.