I ride my black C'dale with this little guy, and I have one in white for my white commuter. As 1nterceptor notes, it's required for NYC, but be forewarned that those tiny bells come with tiny decibels. (Get it!? oh god, sorry for awful pun...)

Personally, I find whistling loudly and shrilly work better for me because I can create that sound much faster than taking a hand off the handlebars and ringing the bell. However, a bell is much more polite and recognizable, and I'll always use that when the Hudson River MUP gums up with traffic during summer commutes and I take it easy for safety's sake anyway. When at speed, I'll whistle away and give a hearty "on your left."

If you like that idea, practice your whistle and get a small inconspicuous bell for the law's sake. Otherwise, dahut's right. Get over the roadie notions and wear your bell proudly: the crazier the design, the better!