It does not currently work with Gmail accounts, which is a significant shortcoming for many potential users.
The Uploader allows the selection of one, some or all of Mail's mailboxes, and of mbox and Maildir folders.
Mail properties (eg, unread or draft) are optionally preserved where possible. Mailbox names can be used as Google Mail labels, and an additional label (by default, the date of uploading) can be added.
The idea of labelling with the upload date is to make it easier to roll back the upload if things go wrong.
Although there is no limitation on the number of messages that can be uploaded at a time, upload rates are throttled to one message per second after the first 500.