I'm a huge fan of OpenBSD, having tried freebsd and most of the major Linux distress before settling on it. If I had to identify the why, i'd have to agree with the jazz reference. I seriously started comparing the various distros around 2003, and OpenBSD just gave a more consistent, well documented and clean experience. It is a very conservative distro, and has a pristine configuration and network stack. It's reliable, and trustworthy. On the downside, it's not particularly performant, nor is it well supported by enterprise Applications - you won't be running oracle 10g on OpenBSD.

Its upgrades are rolled out like clockwork, and are always evolutionary improvements on the previous version.