Earlier this year, on a friend's advice, I started using Style Master as an editor for CSS style sheets and initially was pleased but over time became increasingly frustrated with its clunkiness and slow responsiveness. Recently the same friend said he had switched to CSSEdit because the new version 2 had really improved and he liked it better than Style Master.
So, I decided to give CSSEdit a try. It is great! It is very responsive and quick to update its preview screen. The auto complete feature quickens the authoring of style sheets. You can also open any page on any site and play with the style sheet code much like Firefox's Developer Tools plug-in. If you have dual monitor setup, you can really fly through the authoring process by parking the preview screen on one screen and the code screen on another. Finally, you can organize the code in your style sheets in groups for easy navigation.
You can get a demo version of CSSEdit here. If you decide to purchase, it is only $30.