I just installed it and tried a few bad URLs that were close to being
correct, and it worked very well.  As noted in the Webmaster Tools,
the widget did not produce a link to my HTML sitemap, perhaps because
the filename is "sitemap.htm" instead of "sitemap.html".  I hope that
the widget gets a bit smarter about sitemap filename extensions soon
(.htm, .php, .asp, etc.).

The Webmaster Tools page for installing this widget should also
include some advice to add a complimentary <noscript> section to your
Custom 404 page so that you can give users running with JavaScript
disabled some appropriate advice for finding their way to the
information they're after on your site.