OpenBSD is great on security. But I simply can't use it on a multi-cpu, multi-core system and expect to squeeze excellent performance out of it. I've seen OpenBSD crash to it's knees when running performance testing.
Now in terms of security, a good administrator could secure just about any OS. Heck, you give me a Windows Server 2k3 box, and I'll make it one of the most secure boxes in the world by pulling the cat5, locking it in a cabinet and melting the key down.
So it's really a matter of what you're using it for. Need a high performance MySQL server? I'd go with FreeBSD. Want a desktop? Well personally I'd still go with FreeBSD just because it's what I'm used to and have been using for years, but OpenBSD is a viable alternative on the desktop.