The main criticism of the Hornit I've read is that it does not sound like a motor vehicle (MV) horn. A lot of motorists might think that odd tweety sound is something other than a horn. In fact, it might further distract the motorist who is already in the process of backing out of the drive without looking, R/L hooking you, etc.

The only thing many MV operators respect is larger MVs. As a bicyclist, I think you get the best results if you can look and sound like a MV. Use bright headlights, and get a horn that sounds like a Ford F150 etc.

We all have a built-in horn; most folks recognize the sound/meaning of a scream.

http://www.youtube.com/watch?v=D2_wnQBKnj4