I have always liked FreeBSD. IMHO it has the best and most up-to-date software collection in the form of ports (much more current than Gentoo last time I looked). I was always irritated by how hard it was to upgrade the kernel and core OS. Talking to several friends I found that the new(ish) freebsd-update utility seems to have passed a lot of people by.
It allows binary updates to the FreeBSD OS (not ports or packages) it also supports major and minor release upgrades. Along with portsnap and portupgrade FreeBSD is now as easy to manage as any other OS.
One minor gotcha is that it only updates the version number reported by uname if the kernel has been upgraded.