I currently use NixOS for a personal server (running on a Kimsufi Atom box at €10.20 a month). It's seriously awesome; I have my full operating system defined in a config file which can be versioned and backed up. I can write extra packages and custom services easily, as well, as part of my configuration. It even has its own declarative cloud management tool, NixOps.

One issue is that at present, the system is very much rolling release with snapshots every 6 months called "releases". Unfortunately, there's not currently a good release management process; 14.04 was essentially managed by asking everyone not to merge anything major in the month before, then a week(?) of testing on a separate branch. If there's anyone with release management experience, I'm sure they'd appreciate some help.

The IRC channel on Freenode is extremely friendly, for anyone who has difficulty setting it up.

Someone else on here mentioned Guix; Guix is essentially Nix with a different language for defining packages. Nix's language is purely functional and lazy, but actually really simple; the nixpkgs library defines enough helpers that the vast majority of use of the language looks like a slightly more complex version of JSON.