SASS is amazing and I don't know what I'd do without it.
SASS is a fundamental part of our front-end architecture. I mostly use variables and nesting, but I've also used loops and mixins primarily for animations.
I centralized our color variables on an internal server so that no matter what developer is working on which site, when they compile the SASS files, the colors are standardized. As a general principle, I try not to use any hex codes.
With the colors centralized, I'm able to easily apply global tweaks for accessibility. For instance, if our primary color is too light behind white text or logos and/or icons on a particular component, then I can use SASS' darken function to increase contrast across the board.
Although SASS is loaded with useful features, one thing I really love about it is using it in conjunction with Gulp. With Gulp, I can grab all the SASS files I need and spit them out into the tier system. Finally, I can use the Watch command so that I don't have to manually compile everything.