I have, and from what I've found it has limitations of its own(namely, paginated articles will not work). I've also tried greasemonkey/stylebot scripts, which work great with the caveat that they're on a case-by-case basis. Really all I want is a damn simple way to invert the colors of any website, so white-on-black sites don't burn the eyes when reading at night. I'm not at all bothered by choice of font(unless it's really obnoxious and actually hinders reading of course), and changing the size of text is already easily addressed in modern browsers. Unfortunately Readability had extremely limited options for color(only -one- color scheme had light text on dark background) and font size.

Personally I see this as being in the realm of browser vendors - hopefully we will start to see them include such a feature, and flesh it out in such a way that it's customizable and really works for any site.

I think it's great. When I don't have it, I use and recommend Readable, which does about the same thing that Readability used to do except without all the redirect shenanigans. Readable is awesome.