My reactions to the English voices heard thus far:
Garland: All we heard was a growl, so it's hard to judge. I'll reserve my comments for now.
WoL: Wasn't very impressed. Sounded like your typical stock hero voice you can find in any B-grade modern RPG. I was expecting it to be a bit deeper, also.
Cecil: He made me laugh. Waaay corny. He made me think of an infomercial guy selling hallowed light. "You don't need a cabinet full of cleaners! Got some darkness? I can turn darkness into HALLOWED LIGHT! ;D"
Firion: Actually, based on what little we heard, I thought he sounded excellent. The actor sounded as though he was taking the role seriously and not sounding all peppy and kid-friendly like I've heard countless many do. If the rest of his lines are as well-executed as that one, I'll be very pleased.
Cloud: Not much to say, sounds like the same Cloud we've been hearing for awhile now.
Kuja: Didn't hear quite enough to make a firm judgement...but it doesn't sound too good. The actor seems either inexperienced or not taking things seriously. The voice itself isn't neccesarily bad for Kuja, but the way the actor executed that line certainly was.
Zidane: Well we only got 2 words, so I'm hesitant to say anything. But they were good. I expected him to sound a liiiittle younger, but it wasn't bad, can't complain.