A study of 10,000 civil servants in the UK indicated that bicycling 20 or more miles a week reduced the chances of coronary heart disease by 50%. Not insignificant.
This perplexed me a bit: "The popularity of cycling in the United States as a form of exercise or as a recreational activity is marginal." Maybe it's just my little corner of the world... but in these parts I see a lot of "recreational" cycling (mountain biking, cruising the greenbelt, etc.); it's just hard to get those toy-bike riders to envision how rewarding it can be as transportation.