I’m pleased to announce that you can now get Sass rolled into _s by simply checking the box on Underscores.me. The community has driven this change through pull requests and forks.
It’s taken a little while, but we wanted to do it right. As with the rest of Underscores, we wanted to keep it as simple as possible, offering any extra scripting with a checkbox option rather than imposing it on all developers. Not everyone compiles or uses Sass the same, so _s shouldn’t force anyone to follow one path or another. In this sense, the Sass provided takes a pure approach, not requiring Compass or any other scripts.
Worth noting along with this addition is that the Github version of _s is now purely for development. We strongly recommend only using Underscores.me to download _s, going forward.
Just like with _s itself, the Sass it uses will probably change and evolve with time. What is in place now is a structure, a starting point. Any issues, or requests can be posted on Github, and you can even roll your own using a fork. Just like _s is your theme’s starting point, you can take the Sass in any direction you want.
I hope you are excited as I am to see Sass in _s! I’d like to thank the following people – without them this would not have been possible. As this was a Github project, here are their Github usernames: @gregrickaby @bradp @hugobaeta @obenland @sabreuse @MichaelArestad @jacklenox and myself. I look forward to seeing what things people build and where they take Sass in _s.
ThemeShaper 
Awesome update. Thanks to everyone who worked on it!
Great news! About to start working on a new custom theme, will definitely be making use of this upgrade!
This is gonna be pretty cool. Downloading this upgrade now!
This is a splendid update and will help us save so much time! Thanks to all the contributors!
Thank you, gonna be up with this!
Awesome, super excited for this question. One workflow question. . . I would normally like to fork _s so that I can pull in (or at least check) any upstream changes to make sure I’m integrating any improvements that are applied to the original _s repo. You strongly suggest just using underscores.me as a generator now, however it seems like this will make it impossible to do what I’m describing, no? Do you have a recommendation for a better way to reliably/safely pull in upstream changes without having to start over from scratch with a new underscores.me generated template? Thanks!