Hi folks,
I'd like to throw in some real world experience: my IMAP server runs for just a few users, but they have huge maildirs (>1GB each) with hundreds of folders and, in some folders, thousands of messages.
Before switching to dovecot, courier-imap handled the backend and I used Squirrelmail as the front-end.
imapproxy had a huge (positive) impact on performance, especially when browsing through folders with many messages. Startup (building the maildir tree with message counts) still took its time, and searching in Squirrelmail also was a pain.
Thanks to Dovecot, startup and search (in from/to, subject) now is really fast and I turned off imapproxy completely as it did not further improve the webmail's performance. I guess in environments where authentication is expensive (slow) imapproxy sure is worth a look at.
Best regards
-hannes