It was a toss-up between KH and Xenosaga when I was looking for a new RPG series to get into last year, and Xenosaga won out. For anyone who loves Batou, Xenosaga is a must-have purchase, as Batou's English voice actor is also the voice of the cyborg "Ziggy" (who is both my personal favourite cyborg ever, and my favourite RPG guy ever)!
Batou's voice actor makes a VERY prominent appearance in KH2.
I nearly beat Xenosaga, but at the end I got stuck and gave up. I was really really dumb and missed a weapon attachment for KOS-MOS that came in an email. >< Mind you this was back when I thought 40 hours was way too long to play a game.
As for Aeris's voice actresses, since she has two, I actually didn't mind them. Even though choosing Mandy Moore as a VA is quite silly, I think she did a nice job.