Eugene Fedorenko talks about how Beanstalk refactored the whole app to use SASS. I'm sure a lot of folks are in this situation right now. You want to use SASS, but you have a big app and it's a big move. Short answer: yes.
The only thing I'd suggest they do differently is use Compass. I've also been involved with big SASS rewrites, none of which used Compass, and I highly regret it as: 1) inconsistent mixins across projects blows 2) those mixin files won't be as well maintained as Compass is.
Mixins are one of the most useful and compelling reasons to use a CSS preprocessor. CSS3 is a classic example but we already have those available in any project through Compass. In this post I share some examples of custom user-authored mixins that are useful to specific projects.