Not just that. The ability to enter a sandboxed shell environment, play around with as many packages as you can get your hands on, and then exit back to my clean machine is my absolute favorite feature!!!

SO many times in the past, I'd be concerned about installing new things because they'd break my otherwise pristine developer environment. Yes, rvm/nvm/virtualenv and others help, but they fail to hold a candle to the nix-shell!