Not often, but I'm a software dev, I like things to be reproducible. So I always install everything through "setup brew" ('setup' is my program), and then the only part of my setup that is particular to a new machine is my bootstrap, which is like 30 lines of code:

https://github.com/kbd/setup/blob/master/bootstrap.py

Also, ever set up a new machine and then for days you keep finding system config that's not how you like, or things that aren't installed that should be? I even set my Mac's settings (defaults write ...) through my setup program. I started a new job fairly recently and my system was up and how I like it in one step.