I don't have a bell on my commuter, but I think I want one. I'm not sure why though...as I never encounter pedestrians or other cyclists anyway! I am tempted by the compass one because I have no sense of direction. It would come in handy (I carry a compass in my wedge bag).

I probably wouldn't bother with one on my road bike for the same reason (don't have a need). The only people I ever encounter are on the sidewalks while I'm on the road. I do pass other cyclists, but they understand 'on your left', so I've never had an issue.

If I ride on MUT's, I only do it during 'off hours'. Around here, they are WAY too crowded to ride any other time (unfortunately). I do see how a bell would be a huge benefit in crowded cities or towns or in places where lots of people are using the same path. If I rode in places like that, I'd definitely get a good bell.