They don't weigh much, are charming, less rude than "on your left" and make people smile. I think all bikes in Chicago should have one. Two of my bikes have bells (including my road bike) and I can hardly believe I used to ride in the city without them.
Who agrees with me? What's your favorite kind of bell?
Mine are the Japanese brass ones, though the ones with a built in clock and others in the shape of a tiny teapot are rad too.
Go out to your LBS and get a bell for each one of your bikes!