It steps you through the whole process and makes it happen.
It can and will import *ALL* existing mail from you current mail server... provided you let it have access to the account.
Also, you can have you users move mail themselves if they so choose... with two accounts and one e-mail client, drag and dropping folders etc...
Its not that hard.
You can have GMail administratively transfer the mail through the dashboard as well... but you have to know the user's passwords on the "old" system.