Here is the current feature list, for those of you too lazy to click through:
- Copies every single message from every single folder in your IMAP server to your disk.
- Does incremental copying (i.e., tries very hard to not copy messages twice).
- Tries to do everything as safely as possible (only performs read operations on IMAP)
- Generates mbox formatted files that can be imported into Mail.app (just choose "Other" on the import dialog).
- Is completely and utterly free (distributed under a BSD license).
- I do not support it, or intend to reply to any e-mails regarding problems in it.
And please, do the sensible thing and try it out on a test account first.