I like Willa Holland. She did a good job, especially since it was her first time voice acting. ^_^ That said, I DO think they could have casted someone else. Personally, I would've picked Cristy Carlson Romano (Kim Possible) or Kari Wahlgren (Lady from the DMC series) as Aqua, but Willa did good. ^_^
Jason Dohring was okay as Terra (and utterly epic as him at times too), but I would've casted Crispin Freeman as Terra, personally. He would've been AMAZING as Terra. <3