I don't mind having characters set into classes, but I do feel this is possible to do while leaving in a strong character customization element. Tinkering with party set-up is something I really enjoy, so if that is eliminated I tend to get pissy. But if you take a game like FF9 which had permanent classes but still had you choosing support abilities to shape the class was a good compromise (I would have gone more towards customization, but that is just me.)

If I have to chose between the two I go for blank slates, but I don't see any reason you can't do both.