Check out this interview with Vincent Connare, typographer and former Microsoft employee, aka: The Comic Sans Guy. He makes a compelling argument for Comic Sans, and an also brings up an excellent point about the usage of anything designed:
Type should do exactly what it’s intended to do. That’s why I’m proud of Comic Sans. It was for novice computer users and it succeeded with that market. People use it inappropriately: if they don’t understand how type works, it won’t have any power or meaning to them. I once heard a guy at a Rothko show say: “I could have done that.” He clearly doesn’t know anything about art. He’ll probably use Comic Sans without realising it’s wrong in certain circumstances.