There is a preview option which takes the guess work out.
Here is the setting interface:
After selecting the settings, drag the bookmarklet into your bookmark toolbar. To activate, click on the bookmarklet when it's time to clean up an article.
To see it in action, I've included images of an article before and after applying readability.
Article taken from the NYT: http://www.nytimes.com/2010/12/14/science/14angier.html?ref=science
As one can see, applying Readability cleans up the article substantially and although not visible in the image above, it provides the option to email and print out this version.
Curious about how to clean up youtube clips? Then check out my post on Quietube: Video without Distractions.