NixOS 14.04 “Baboon” has been released, the second stable release branch. It brings Linux 3.12, systemd 212, GCC 4.8, Glibc 2.19, KDE 4.12, light-weight NixOS containers, and much more. See the release notes for details. You can get NixOS 14.04 ISOs and VirtualBox appliances from the download page. For information on how to upgrade a 13.10 system to 14.04, check out the manual section on upgrading.