I never watched the anime, so my only exposure to this series, other than the original manga, was the recently ended internet radio. Basically, I didn't know what to expect.
I think the casting is very good, for the most part. In fact, now that I think about it, Kate is the only one that just stands out in all the wrong ways. I don't think a young foreigner like Kate would pick up such a messed up Japanese. She's really an eyesore, or rather an earsore, to listen to. And as nice as Hanazawa-san's voice is as Sora, it's all wrong because real Sora wouldn't talk much at all. She's just that kind of a character, and it'd be practically impossible to portray her properly in an audio-only drama, so she's not portrayed properly.
Illustrations in and around the CD (jacket, booklet, etc.) are excellent. I'd almost recommend the CD because of this. But then, realistically, you'd be better off just picking up the art book or something.
Sound quality is excellent! Even the last vocal track appears to be left alone. Somebody must've forgotten to press the evil "mess up this master" button because they thought this CD only contains drama tracks. I hope they're always so forgetful! A forgetful mastering engineer is better than an incompetent and/or principleless(sp?) engineer, at least in this case.