Well I've finally decided to take the plunge. I'm going to convert to Maildir for my mailboxes tonight. The impetus has been Squirrelmail, which Holly uses for her email. It needs an IMAP server and I've now been through UW and Dovecot IMAP servers, and both of them are now silently failing after upgrades. No errors given in logs, no reason given.
So tonight, armed with mb2md I'm gonna make the big switch, then install Courier IMAP. Hopefully it'll work, and hopefully it'll be a bit faster too.