I picked up my IP6 64 GB on release day and upgraded from a IP5. The things that have become very obvious to me after a week or so of use are the significantly improved battery life, the huge difference that the decreased thickness makes in terms of the ergonomic feel of the phone in the hand and in my pockets, and that I was wrong in previously dismissing the benefit of a bigger screen.
In terms of the improved battery life, I am not comparing a 2 year old IP5's battery life with my new phone but rather the battery life of my old phone when new versus the 6 -- doing any kind of browsing that involves the use of cellular data would lead to massive and precipitous drop in battery percentage on the 5 (around 1% every 2 to 3 minutes). On the IP5, doing no more than 40 to 60 mins of Prismatic and news web browsing a day (on mobile data), 1 push e-mail account, a bit of texting and about 30 to 40 minutes of calling, and perhaps a couple of hours of music playing would have guaranteed the need for a recharge at the end of the day. OTOH, with the 6 I am getting around 1.5 to even 2 days worth of similar usage before I need to charge my battery. And the battery level drops MUCH slower when the phone is on standby compared to the 5.
And I disagree massively with those people who say that the thinness of this thing is just a gimmick -- Apple (and perhaps to some extent HTC) seems to be the rare brand that understands the need for a device to feel pleasant to hold. Its feel "in hand" is vastly superior to my IP5.
The only thing I don't like about this thing is that one-handing it isn't as convenient as the 5 but overall I love it. And hopefully some of the laggard developers will start updating their apps to work with the 6 better. Ugly, blow up fonts are uncool :(