I'm a big fan of bells. If you ride a lot on busy multi-use paths (MUPs) I almost think they are essential. Yelling "on your left" is fine, but it gets old if you have to do it 100 times on a single ride. Plus I feel like a gentle sounding "ding ding" chime is a friendlier notice than some dude yelling "on your left" in between his gasps for breath as he pants. I've passed and been passed by both methods thousands of times and I prefer to be passed by a friendly "ding ding." It is a sound that is eternally linked to the bicycle (cue Queen's 1978 opus: "I want to ride my bicycle...I want to ride my bike...") and it brings the joy of this efficient beautiful machine to mind every time I hear it.
I kept my trek bell. I like it for its simplicity and am glad they include it by default. However, I do have one major complaint about the Trek bell. The design does not work well in the rain at all. When water drops accumulate on the bell, it has a dampening effect and does not ring out as it should. When wet it sounds more like "click click" instead of "ding ding."
I've had other bells (I think it was the incredi-bell) that worked pretty well in the rain. But I like the mechanism of the Trek bell better than the incredi-bell. One day I may splurge for a spurcycle, but I just can't bring myself to spend that much on a bell yet.