Alright. So the game's out. I've been repeatedly trying to sum up how I feel about this game from pages and pages of tl;dr (which I have typed out in full, mind you) to a couple of paragraphs. This is the best attempt at distilling my thoughts into the core essence of what I think about the game.
The cast sells the game. The end. It's about a user experience that's more than the sum of its parts. It's basically a game about going on a road trip with these four idiots, and anything outside of that, even the bombastic storyline and set pieces typical of a Final Fantasy, is a secondary backdrop to that.
It had two big things going against it. Firstly, the previous mainline game had a cast that, while unique, wasn't very well liked. Secondly, open world games do not do this. It costs too much resources/effort to present a fully voiced and realized cast this way. The most you usually get are a bunch of practically interchangeable companions (which is part of the concept of most open world games - give lots of choice, let the player make his own story) Going from superficial - and sometimes romance-eable - relationships like those to a group of four that constantly interact
as a group as well as in pairs for the myriad situations in the game is an absurd jump in complexity. Fighting these two things should have been an uphill battle.
However, the cast just...
works. And it wouldn't work if they'd met along the journey, much like every other JRPG (proper JRPG, old school JRPG, whatever you want to call it). It works because they're already friends, they already have a backstory and history together, and those bonds are tested in various ways throughout the journey.
I'm not saying people aren't justified in saying the overall narrative may feel a little diluted or weak, or that the supporting cast aren't utilized enough, but even all that aside, the main four sell the experience of a road trip enough that I personally feel like I can overlook it (despite being a 'mainline FF', whatever that means/entails) because I don't think any other game has sold me the experience of simply being on a road trip as well.
Also Prompto's pictures are the icing on the cake. 6 inches of icing on a 4ft tall cake.