The lackluster plot worries me, playing through the Kingdom Hearts games and a couple other older titles were made increasingly frustrating due to naff storytelling. However, if it takes a backseat then that could negate that issue, my general problem was that in the aforementioned titles the poor narrative is unnecessarily dragged out, too prominent.
The storytelling isn't really naff per-se, it's more that there isn't enough of it. You'll occasionally be doing something which seems vitally important to the plot but you're not entirely sure why
you're doing it.
Point in case; one of the central motivations for the main "evil lead" as it were, is only explained through an animated movie that plays before the title screen and nowhere else. Which is fine perhaps, except, it's one of 3 possible movies it plays. So unless you sit there and watch them all play, you'd completely miss out on it.
It was one of my biggest gripes I think when I finished it because when you actually got the little snippets of plot, it was actually really interesting.
For a lot of people though, the combat and some of the challenges in the story and non-story missions was enough to make the game very worthwhile to play. Once again; a bit like FF13 in that respect.