Main reasons to use a bell especially on shared cycle/pedestrian paths - safety and consideration. I think my own experiences as a pedestrian illustrate why using your bell is a good idea.
When walking, I now try to look over my shoulder before crossing the path for any reason. The other week, I was walking along when someone on a mountain bike came whizzing past. Had it hit me it would have hurt...if not put me in hospital...
Also, have you ever been walking along and you're suddenly aware of a bike trying to push past, hearing only a squeak of the brakes or click of gears changing? Startling and annoying, I find, especially given they cannot be bothered even verbally, to let you know they're there. Try not to be that cyclist...
I like to ring when at least 10 metres behind the pedestrian. This has gone down well and I've been thanked on several occasions: the pedestrian pleasantly surprised at my courtesy.
As well as enhancing safety and consideration, bells can be complementary to an urban bike's looks - it seems incomplete without one. They're generally inexpensive, ranging from under a pound to around £40 for one made from titanium:
Basic flick bell - these come in many other colours:
Different: Source: Flickr/J Ferguson - this type isn't actually unique - but I haven't seen them outside of the net. I would have thought they're less ergonomic as you would have to take your hand off the grips. Adds variety and style though - which being individualistic, I like. Something else on the shopping list.
