This is a pleasant change. My goto framework for internal apps is Bootstrap (no surprise there) although I actually end up using only a handful of elements. Flakes looks like a good enough collection of elements to cover 95% of my use cases. Definitely going to use this in my next app.
Why single out the SASS version? If you compile LESS files you have the same option. I also think Bootstrap has a form on its website which compiles the CSS with the components you want, without having to install/run a preprocessor.
I've always used the Less version myself as a base for customization... lately, I'll npm it in as a requirement... copy out the bootstrap.less and variables.less into my project, update the references and work from there. It's relatively straight forward to do so... from there, I can comment out the parts I don't use. variables.less is mainly copied as a reference point... In my app.less, I include the bootstrap variables, then my own variables, then the rest of bootstrap, then my application less files.
At this point sass doesn't really offer much over less (other than a slightly stronger presence in the design community). I'm using node mostly, which fits in better for me, and makes less a better/tighter fit.
For better or worse, being able to bring in less, react and the like combined with SPDY/HTTP2 will bring some interesting development as things progress. Right now transpiling ES6/7 stuff to ES5 is probably the biggest bottleneck in terms of serving applications closer to as-is.
Thanks for sharing!