In the city, (Boston, Cambridge) most people are commuting wearing regular clothes, or casual cycling clothes. Here, almost everyone is in lycra, I'd say 90%. And, there is a large number of commuters. I'd say some of this is because the commuters in the suburbs have a longer commute... some of them up to 20 miles, even if it's suburb to suburb.
Personally, I cannot stand the feeling of riding in any type of regular clothing. I sweat really easily. Even when I had my 6.5 mile commute, I'd be drenched when I arrived. So, showering before the commute was OK, but I would have to do a serious clean up when I got there. The thought of being in sweaty clothes all day, yuck.