A recent convert to Dovecot, from Courier. I host mail for many virtual domains in a Postfix / Dovecot POP3 and IMAP w/TLS, SMTP Auth environment with remote clients and SquirrelMail (under review) running on local or nearby web servers. FreeBSD everywhere.
My motivation in moving to Dovecot was more simplicity; certainly implementation is simpler than Courier and its pieces. Performance boost for IMAP is nice as well.
Am currently in the process of rewriting my hosting software tools to move away from Postgres client databases for simple things like user id lookup and mail data to plain old text files, not sure of the scalability but from testing its clear these will meet our needs for the forseeable future, and remove a big chunk of software (and the resources it gobbles) and yet another dependency.
I wasn't able to puzzle through why xargs on FreeBSD wouldn't work in the sh script courier2dovecot, so I wrote my own in Python: http://www.aucontraire.net/scripts/mail/courier2dovecot.py