i'd go with thunderbird - it's a no brainer to setup, drag 'n drop. hell, i even used to use it to import email from one email server to another of a different kind with imap on both (lotus -> exchange, groupwise -> exchange). you can also tell thunderbird to store in mbox which is real cool. oh, and gmail's imap converts your labels into folders, so that's seamless (well, it'll copy multiple if you've got something with multiple labels). fwiw