"My main use for OpenBSD is as a firewall/router, and they frequently make breaking changes to the pf rule syntax so I had to abandon it."

Uhm... well, I've been using it since 3.0 (when pf was introduced) and its been very stable. You have at least 6 months to convert your pf and its never taken me more than a day to get it changed. Its well documented and frankly, even with all the horror stories, I've found friendly help each time I had a question (do read the manual before asking a question). Looking at your firewall rules every 6 months isn't exactly stressful.