Yeah, I thought about it when I wrote that comment, and I cannot figure out what the killer feature is; I just do all my CSS editing by slurping up application.css into CSSEdit, making things the way I want them, and then c-p'ing them back into application.css.
I spend a bit of time in Firebug too, but I never use it for CSS editing.
I guess one obvious great feature of CSSEdit is that it's immediately apparent that you can use it to... dynamically edit CSS. It's in the name and everything!
Whereas I'm not sure I'd have realized that you can edit the CSS in a Firebug document if I hadn't clicked on the right-hand panel accidentally one day. Or maybe that's one of the things I picked up from watching John Resig demo something. They should ship a copy of John Resig along with Firebug.
If I remember right, CSSEdit also has a generally clearer UI than Firebug, and of course it actually saves your experiments rather than requiring you to switch to emacs to make them permanent, which might actually be a bigger workflow improvement than I imagine. Maybe I'll try it out some more.
My Firebug addiction is at the point now where I can barely think about CSS without firing it up. The most painful part of a project is still IE-compatibility but no longer just because of general quirkiness - it's more about being forced off of Firebug to make edits.
I spend a bit of time in Firebug too, but I never use it for CSS editing.