It's legal in my city to ride on sidewalks in residential areas, but people do it all the time downtown. I used to be a lot more bothered by it than I am now; these days unless they are going really fast or otherwise riding unsafely, or unless the sidewalk is heavily traveled or right next to a big clean bike lane, I cut them some slack because I know some people would not ride at all if they had to be in the street. And construction and stupid traffic plans have cut out some of our better bike lanes, so there are some cross-town trips that either involve taking the lane on a busy three-lane arterial, or taking the sidewalk. Most people who use bikes for transportation in this city ride single-speed cruisers so I don't blame them for taking the sidewalk, especially since they mostly behave like pedestrians (riding very slowly, stopping at crosswalks, etc.) It's their necks.

That said, the ones who really irritate me are the people riding fixies without brakes ... on the sidewalk. I see this all the time and I don't get it! You are so cool that you ride a fixed gear, and you don't even need a brake, but instead of taking the lane you are speeding on the sidewalk and weaving around pedestrians? There is one guy who does this even when there is a bike lane or a low-traffic street, and he rides like a maniac, too.