People make the most interesting photographs. However, it’s impolite, and maybe even unethical, to photograph people without their permission.
Famous photographers like Henri Cartier-Bresson and Walker Evans have no qualms about this sort of photography. Nor do serious photojournalists.
Lots of local photographers, like Troy Holden and the folks at the All City photoblog, take pictures like this. I guess their blogs would be a lot less interesting without them.