If Sass (.scss) is used correctly, I find it to be a great way to theme the CSS through variables and calculations. Also, it's smart, on huge projects, to create partials and selectively import them into different parent files so you're only serving what's needed between features, public and private pages of a site.
Then, you can write a conditional - used with the same library you create - that will output SEO-friendly "above the fold" CSS that you can add inline (so it's not pulling another file, thus lowering your score.) I've also made mixins that handle cross-browser issues with vendor prefixes, etc.
It's nice to have a dynamic way of writing CSS, in a nesting fashion. Just like anything else, it can be misused or overused, but for those of us that don't do either, it certainly speeds up my process since I don't have to continually type parent elements over and over again to keep a certain class exclusively within its scope. I feel it also helps me organize everything more effectively.
The two things I recommend avoiding would be; 1) Bootstrap, 2) Any "Key to the City" Framework. They are a complete waste of time to learn, imo. Write it yourself, so there's a deeper understanding of why you would use Sass as opposed to using something that would automate a process, while bloating it for no reason. Also, avoid using@extend.