Demetri is one of the most original and funniest stand-up comedians on stage today... and every time him or John Hodgman are on the Daily Show it becomes an instant classic... I loved his "expose" on MySpace... as well as his consumer report on Xbox 360s
If you've never seen him outside of his appearances on the Daily Show, get a copy of either of his two specials HOWEVER YOU CAN...
also for more of his low-key, subdued comedy stylings, check out http://www.clearification.com/ its a site he designed with Microsoft for the big Vista launch last year.
His stage presence reminds me of Andy Kauffman playing Mitch Hedburg in a film directed by Wes Anderson... only better