I can say that configuring opensmtpd is surprisingly easy, comparing to postfix or exim. My config is 13 lines, and it supports dovecot integration, DKIM signing of outbound messages, TLS and authenticated submission. I wish every server has so simple configuration. But that's only SMTP part of mail processing.
I've became a big fan of OpenSMTPD lately, although for two specific use cases. I manage a number of mail systems, some small and some large, and used Postfix exclusively.
Recently, after discovering and playing with OpenSMTPD, I've started replacing Postfix with OpenSMTPD on null mailers and internal relays. My Postfix configuration is pretty much identical on all of those hosts and I've gotten it pretty polished over the years, but there's something about the simplicity of OpenSMTPD that I really like.
I don't envision moving from Postfix anytime soon on my main mail systems (which serve many, many domains and users). If I had a much simpler mail setup I would definitely consider it, though.
