Yes, Maildir stores each email in a separate file whereas mbox uses on big file to store all messages. That means if an email is corrupted you lose all your mails if you use mbox.

And if you use mbox each time a user fetches his mails over POP3, the whole mbox is copied to another file from which the mails are retrieved. This means that if a user has a quota limit, this limit has to be at least twive the size of his mbox file. This does not happen if you use Maildir because then no files are copied.

Thomas