I seem to recall that Readability required transmitting the page I was viewing to their servers, so I found myself avoiding it. I don't know if there were good alternatives, but I do have faith that Mozilla's will be good (although I haven't tried the nightly or anything), so I'm excited about that. Once I used it for a while, it felt like something a browser should just have.
We've put the source we're using up on github ( https://github.com/mozilla/readability ) and plan to use the same library on all platforms. Any contributions are welcome.
(I'm sure others have made similar remarks about other features that were added over time that were originally in extensions)