Been using it for a few years now, it's freaking amazing.

• My laptop died, recovery was: 1. rebuild my system from git 2. load the backup data. Took me about 20 minutes to prepare everything, went on with my day and when I got back to the laptop it was right as I needed it.
• I need to target different Python version, with packages and everything, on different systems. Using nix I can a git version of a project to a Python environment, and it just works. Same with other languages.
• Rollbacks are epic for experiments.
• I can duplicate shared settings between computers, for example, I have a desktop PC and a laptop, both run the same DE, browser, email client, etc, all those goodies are defined from the same file. My server runs no DE, so its configuration file does not load the PC nix file.