Every preview of the game in English has stated that the voice acting is overall very weak, particularly for the main characters. And from the handful of gameplay videos I've seen, I would agree entirely (although some of the side characters sound pretty decent). With all the promotion for the Japanese voice acting, however, as well as from what I've heard in the trailer, it doesn't sound nearly as bad.
To those who've played the Japanese version (or better yet, those who actually understand Japanese), what is your opinion of the voice acting? I would be extremely disappointed if they only screwed up the VA in the English version, after IV and V turned out so well (even Tactics sounded better than this, and it had its own share of voicing issues). I've been considering getting the Japanese version, but if there really is a huge difference in voicing quality, I may get it after all even if I can't really understand it.