I put the trap out again for the year not even five days ago and it's filling up!
I've found it does catch a lot more greenheads than horseflies, but every single dead biting fly is a victory. Totally worth the few dollars I spent on materials.
A friend who horse-sat for me did make a funny comment, that my horseflies seem smarter than hers at home. This made me a little worried that my trap plus natural selection will cause the evolution of a super-intelligent population of horseflies. Eek!