Take a look at NixOS. I've been using it for a while and it's beautiful. Upgrades are very close to being fully atomic, and you can rollback arbitrarily through the boot menu in case a system change—an upgrade or configuration change—ruins something. Moreover, package dependencies are isolated from each other, you can install packages as non-root, etc etc.
Their approach to system configuration and package management looks great. I'm a little bit concerned about what a complete configuration.nix file would look like for a daily desktop development system though, and it looks like they've still got a lot of packages to fix (if I'm reading https://nixos.org/wiki/Zero_Hydra_Failures correctly), but still, looks like a big improvement over other approaches.
I love the configuration format and the .nix files for my laptop setup is very managable. Since I have the same hardware as my brother, we've set up a repository with the main configuration with custom files for our own options. So for example, I enabled Avahi ZeroConf DNS (one .nix line) and my brother only needed to pull the repo and run NixOS's "rebuild & switch" command.
The really great thing about the configuration language is that the same dependency tracking used for packages is used for all values in the system... So for example, if you have servers set up as Nix packages whose configs refer to the Nix variable for the system hostname, then if you change the hostname, those packages will be reconfigured automatically.
Yeah, there's quite a bit of work needed on packages, but I actually haven't had any trouble. I believe the "unstable" channel is defined as the latest set of packages that all build successfully together on Hydra, and the "stable" channels are Ubuntu-like LTS releases.
Anyhow I encourage people to give it a go! They have Amazon AMI images you can use to run NixOS on EC2 easily, the community is nice, and you can find lots of example configs on GitHub. Something like NixOS is bound to win as far as I'm concerned. Next level stuff!
