My guess, based on what I've played and seen:
It will be better than the FF13 series but won't come close to the top of the series.
It will be a bloated Frankenstein of a game filled with half-baked ideas, a million verticle slices of sequences stitched together. A big, empty open-ish world with repetitive monster battles. It will have a bunch of ideas and themes it never fully explores. The characters will be cliche as hell, with atrocious english voice acting. The plot will be contrived and over-wrought, like the game itself.
It will have a massive hype launch before the inevitable backlash. The honey-moon period will wane and people will be faced with the game's numerous flaws. The die-hard fans, many of which decided it was the greatest game ever before it even released, will continue to sing its praises and argue loudly on the internet, taking any criticism way too personally.
It will be forgotten after Persona 5 releases but SE will continue to force the FF15 brand down our throats through-out the remainder of the gen, regardless of reception.