After hearing about JETPLOW [0], I decided to replace the Cisco ASA that I had been using for my (fiber) Internet connection at home with an open-source router/firewall.
I had a RouterMaxx 1106 [1] (PDF) laying around so I decided to put OpenBSD on it and use that. In short order, I built an OpenBSD virtual machine, used flashrd [2] to build an OpenBSD image I could put on a CompactFlash card, and had it up and running.
It just works.
About two years later, it's still humming along nicely. The only downtime that I've had is when I upgraded to a new OpenBSD version. I could've minimized somewhat but I use a new CF card every time.
Nowadays, I've also got a dedicated laptop running OpenBSD that I use solely for "security critical" stuff for $work. A separate machine stands in between the Internet and our MX hosts and runs OpenBSD's spamd [3] to help keep spam out of our users' mailboxes (N.B.: the number of messages hitting out Barracuda dropped by ~70%, with zero complaints received from any of our users).
In addition, we're just beginning a project to basically rebuild our entire server infrastructure and we'll be using OpenBSD for certain functions: remote access (OpenVPN), SSH jump hosts, possibly authoritative DNS, etc.
I'm a firm believer in using "the best tool for the job". Most of our servers will be running FreeBSD, but OpenBSD definitely has its place in our environment.
[0]: https://nsa.gov1.info/dni/nsa-ant-catalog/firewalls/
[1]: http://www.balticnetworks.com/docs/routermaxx%206%20port.pdf