I've been running it for a few years now, admittedly with a very basic setup. My config is about ten lines long, written in five minutes or so. I haven't really needed to touch it since.
I love OpenSMTPD. The only issue I had with it was, no filter API (just fixed, as of a month ago or so) and to get things like spam and antivirus filtering, or domainkeys, you have to proxy chain it. Also, the API changed very rapidly, and I had to run nightlies for a while to get around a few issues (I'm on stable now though with no problems.) These things add complexity to the configuration, but I would still rather do this than maintain any other mail server software.
It's on my radar, but to date I've not had (or more accurately made) the time to seriously evaluate it. I'm looking at an architecture overhaul soon, so will make the effort to take a proper look before then.
I've been running it for a few years now, admittedly with a very basic setup. My config is about ten lines long, written in five minutes or so. I haven't really needed to touch it since.
[1] https://www.opensmtpd.org/