Works well (when the site has this info, of course), and really useful. Now I search a date plugin for Google, to know the date of websites in the search page!

The only think I don't like is the "by HTTP Header" that I can't manage to remove. (in the HTML popup mode).

Works fine in my experience and I like the customization options. Keep in mind that probably half the websites out there or more either don't send the Last-Modified header or send a value that is the current date/time. This isn't the extension's fault.