I had a really cute one the I was using with a bell until Macie figured out how to take it off herself... she would either bite the clip until it released or put her foot on the bell and pull until it popped loose. I was afraid she would chew on it and get choked on a piece of the plastic clip- so I retired the break-away collar. I ended up just buying a bell from Petco to put a regular collar that Ms. Houdini couldn't excape from.
A bell is a necessity at my house- all my dogs wear a bell on their collars. When we take them all out for a walk it sounds like Santa Claus is coming to town!