It essentially has a similar effect to running your own IMAP server. With the added benefit that you're able to manage the raw mail yourself locally on your server and run your own procmail (or anything else against it), plus make your own backups on your own schedule, etc.

Dreamhost's new mail servers aren't accessible by SSH so if there's ever a hiccup there, I can lose all my mail. I'm going to have to verify the efficacy of Time Machine backups with regards to Mail's caches copies of the IMAP mail to take care of this.

Some good news at least for my INBOX. I've moved the mail to a temp folder and back to the INBOX using Mac OS Mail and it's fixed the timestamps. The problems seem to have been caused by the IMAP upload utility I used to send the mail up to the server last night. I still can't get Mail to do large batches without failing, but it's a start.