I originally was pretty against the idea when I first heard of it, but right now I'm actually looking forward to the variation it can give all the fighters.
I'm pretty sure customization will only apply to B moves, and that it'll be relatively simplistic; something like a fixed slider with 2 extremes on either side, with the neutral position being the unaltered move (so Mario's Fire Balls are either large, normal or small). I do hope it sees some usage in tournaments, although it depends on the extent of its balance; if some ridiculous combination props up that shuts down other characters on the roster I'd be fine without it.