I find that some of the voice actors performances are made worse when in scenes with some of the Disney character voice actors, as whether they are original or not, they sound as true to how they sounded in there respective films. I personally just found Mena Suvari's voice as Aerith annoying if anything else.
I also found Jason Dohring as Terra to be poor, as I found Jesse McCartney as always giving a good performance as Ven especially making him different from Roxas IMO in the way he delivers his lines in a more upbeat, personable way. Willa Holland as Aqua was also good I think, I personally loved it when she calls Vanitas a 'freak'. Great emoting in that line, you can really hear the frustration and angst she has for him.
I just hope if Aerith makes a cameo in KH3 they replace her with someone else. It'd be great if Mandy Moore came back, and David Boreanaz back as Leon.