It could use more tweaking - I tried to read the latest pg essay with it, but all it would display was the image with the light "PAUL GRAHAM" text at the top of the page. I then tried to go back to the essay, but it looks like the only way to un-Readability the page is to reload it. I then tried to highlight the text that I wanted to read, but Readability doesn't take this into account when generating the page.
When you have a service that guesses what the input is supposed to be, there should be a way to gracefully fail and allow the user to manually specify it, and if that doesn't work it should be easy to disable the service.
If you're a Firefox user and savvy about CSS selectors, I recommend the Stylish add-on (http://userstyles.org/). It lets you create blanket tweaks for all sites and specific tweaks for specific sites. And you can turn your tweaks off and on at will. It works even with Javascript turned off. I set colors and fonts and font sizes, adjust column widths, eliminate entire columns, make images semi-transparent so they don't bug me so much, etc.
Also, if you're an opera user or have greasemonkey the styles can be loaded as userscripts, which will just use javascript to edit the css style changes you have selected.
Being to select the chunk of text you interested would probably solve this issue. I wonder how Readability "selects" the "relevant" part of the page. Maybe it is just a smart heuristic about what the user want ;)
Come from HN => must be a PG fan => Just display "PAUL GRAHAM" => Go back to HN and vote
Reloading the page 'disable' the service, even if it is far from ideal.
I really like it though. I was looking for something similar for years.
The PrintWhatYouLike bookmarklet (http://www.printwhatyoulike.com/bookmarklet) does exactly what you're talking about. You can select the contents of pg's article and the bookmarklet will remove everything else on the page. There are lots of other options to reformat any page to your liking. And everything is undoable.
Yea, there is a trade-off between speed and accuracy. PrintWhatYouLike aims more for accuracy- you can reformat the page exactly as you want, but its more complicated to use and takes a bit longer. Readability is fast, but you have no control over the output.
When you have a service that guesses what the input is supposed to be, there should be a way to gracefully fail and allow the user to manually specify it, and if that doesn't work it should be easy to disable the service.