"Nobody wants big phones".
But yeah I think the Plus will definitely be a major player going forward for Apple, perhaps even becoming the primary iPhone size by the time the 7 rolls around...
I do think that most configurations are "sold out" now. None will ever be sold out, but the initial stock for 9/19 deliveries have mostly dried up and will likely be completely depleted by the early afternoon.
Most people will be able to get all models and configurations in October and November, though. Not a big deal if you missed out on the 9/19 if you have to wait two extra weeks.
As far as I can see, most options for the Plus dried up pretty quickly after they went on order: they maybe lasted an hour at most.