I cut my open-source teeth on BSD, mainly because I had a terrifyingly low-spec PC (pentium 1, 64MB RAM) when new PCs were starting to come with dual-core processors. OpenBSD had very low system requirements and I liked the stance they took on making sure everything was correct. An operating system whose primary focus is on correctness and security sounds pretty good to me!
I found the community to be no-nonsense and having read Absolute OpenBSD by Michael W. Lucas I was pretty clued up and could make it dance and sing as I pleased. As time went on (and I got new PC hardware!) I found I was needing programs that didn't run on BSD so I often dual-booted with Windows. I disliked Windows so ended up triple-booting Windows for games, Linux for a mix of general use and work, and OpenBSD or work.
After a while I just found myself using OpenBSD less and Linux more. I've still got an installation of FreeBSD, but I'm waiting for them to get the UEFI booting sorted before I put it on my laptop. Once that's sorted games and skype will be the only things keeping me on Linux.
I still have a better 'feel' for how BSD systems are laid out, and I've never had a problem upgrading an OpenBSD/FreeBSD system. I've not often had problems upgrading Ubuntu, but it's been often enough that an upgrade has failed that I always feel a bit nervous about it.