They haven't released numbers yet, but the press reports seem to indicate that the 6 Plus demand is outstripping supply yet the chorus of people who think that even the 6 is too big let alone the 6 plus is as loud as ever. I think this is an interesting dichotomy.
I think the 6 Plus is fine -- I find more screen better than less screen, even if the increased size limits one-handed usage. I don't think there's an "ideal" size for any phone unless you toss in some usage requirements, like one-handed use or pocket storage complaints. I know some people who would use a full-size iPad as a phone if they could because none of the one-handed use or pocket issues apply to them. I think it's just a matter of personal preference.
I do think it's interesting that Tim Cook's Apple is responding to market demand instead of imposing a Jobsian design fascism. I also think that for a decent chunk of people, the 6 Plus is meant to take over some of the things they'd use a tablet for. I'm mostly happy with my iPad 3 (even with iOS8), but I think with a 6 Plus I'll reach for it less and put off upgrading it until it runs out of iOS updates.
And I think a lot of people who want both but can't swing it financially will find a 6 Plus a reasonable universal device. This is what surprised me about the 6 Plus release as I'm pretty sure it will eat into iPad Mini sales and even some full-size iPad sales.
What would be nice and I don't know if we'll ever get there for lots of reasons (technological and sales/marketing) would be a watch-sized device becoming the root device with the phone or tablet being the kind of screen/user interface, tethered to the phone for network access. That way you could pick your "phone" based on size preference, or none at all if all you wanted was bluetooth audio and phone calling.