We could write our own Sass @mixins to help with CSS3 stuff (like gradients), but there is a Sass framework that already exists called Compass that already has that stuff ready to go. It takes a bit of a leap of faith to reliquish control of stuff like this (I guess no more than using a preprocessor to begin with), but it's worth it.
Why is using Compass a good idea?
The @mixins are very well written. They match the spec where they can and are very consistant.
Compass updates so you'll stay up to date on current best practices automatically. It will probably do a more comprehensive job of doing that than you can alone.
It has a bunch of other powerful stuff you can tap into, like spriting (which we won't get to in this project but is sweet).
CodeKit works great with Compass (it even ships with it). We lose some of the control we had in CodeKit (like controlling file compiling paths directly) but it's not a big deal. We can still control most stuff from the config.rb file now in our project.
Ultimately we use Compass to @include transitions for the navigation links giving them a softer feel.