As a network appliance type device at least I'd say it's still very relevant. I still prefer configuring / maintaining pf over iptables (or any other competitor I've tried) for any non-trivial ruleset, the documentation is IMO much better than most of the other stuff out there, it's relatively secure and relatively stable, and the performance and compatibility with older hardware has been great (in my experience). I use it for my gateway device and have never had any problems. I briefly used Linux for the same task and found myself spending more time messing with it. I could easily see it replacing all sorts of expensive commercial solutions at my workplace but managers like commercial vendors. It's just well put together and does what it's built for quite well. I think there's room for all sorts of stuff in the "grand scheme", not just shiny and popular stuff.