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.