The !bang syntax is definitely one of DDG's best features. Being able to access a few hundred site searches with a keyword is incredibly powerful, and saves time on creating quick searches in Firefox.
Definitely agree. I set DDG as Firefox' default search engine (about:config --> keyword.URL) and it has come to the point that searching with the dedicated search bar has begun feeling clunky. I think there are no important sites which I frequent that aren't included in the huge amount of bangs yet. Could just as well get rid of the search bar altogether...
(The full list of !bang commands http://duckduckgo.com/bang.html)