I've been using OpenBSD for several years (since 3.4...wow, that's almost seven years), as my primary OS. I had been running Debian for 2~3 years, and Red Hat (briefly, ick) before that. A lot has been covered already, but:
1. The BSDs feel significantly more cohesive to me. Other people have mentioned this, but it deserves greater emphasis. The config files, man pages, etc. are quite a bit more consistent, and my first guess about where something is located is nearly always right. Red Hat, in particular, seemed to cavalierly scatter things in /usr/bin, /usr/local/bin, /usr/local/sbin, etc. (Around 7.0, IIRC.) The man pages & other docs are consistently excellent, for everything from malloc to spamd, it's a cleanly and consistently designed system for learning Unix / system administration.
2. OpenBSD has a focus on security. While this isn't my top priority, personally, I really like its secondary effects - it tends towards minimalism, so there are less dusty corners to harbor exploitable bugs, and they pass security patches upstream. OpenBSD's malloc (http://www.openbsd.org/cgi-bin/man.cgi?query=malloc) can also be configured for very aggressive scrutiny. Not quite the same as running everything under valgrind, but it has also much less overhead since it's better integrated. If you're working in C, it's a great platform.
3. I can install and configure an OpenBSD system in under ten minutes, easy. Its installer is spartan and based on text prompts. Once you know what to expect, it's a snap. Also, they put all recognized drivers in the kernel (ostensibly since debugging weird custom-kernel issues would be tough with a smaller community) - if the dmesg doesn't say "... - not recognized" at boot, you're usually set. No rebuilding kernels or modules just to get things working. I haven't dealt with much really weird hardware, but any random crappy old x86/amd stuff has worked fine. (Cheap old Thinkpads run BSD brilliantly, by the way. I'm typing this on a T41.)
4. The BSDs have less GNU stuff in the userland. This is probably a matter of taste, but a lot of the GNU stuff seems really bloated by comparison - gmake vs. BSD make, gawk vs. awk, etc. (That said, I'm also an Emacs user. :) ) The BSDs also seem less political. It isn't Linux or GNU/Linux, it's just BSD.
5. I really like source / ports-tree-based packaging. You can still use binary packages (especially if you're installing KDE, doing that from source is just silly), but ports+flavors works quite well. Some Linux distros (Arch and Gentoo) have similar packaging systems, now. Of course, some programs are hard to bootstrap - GHC, in particular, has lagged behind.
I've only used FreeBSD a bit here and there, but it also seems like an extremely solid OS, and most of the same advantages apply. It doesn't have as much focus on security, but its community is much larger, and it's probably more newbie-friendly.