When we hiked in Grand Teton few years back a park ranger stopped us and very politely said that bear bells are not effective, they are too mechanical for bears to be scared of them, and in fact sometimes makes them curious. She said that bear bells are prohibited to be sold in Tetons, and while they can't prohibit anyone from carrying it she strongly recommends to reconsider.

Since then I don't carry them - I figured a park ranger should know