We thought about going to them with our new rig but between the gas engine and sirens and airhorn being in the bumper, it's quiet. Our newer Type Is are really quiet as well. But a Ford E450 with the 6.0 Diesel and airhorns mounted under the seats is just too loud. Which is what I was in at AMR today. As soon as the fan comes on it's impossible to hear.
I do love my airhorns though. Maybe not a must have piece of equipment, but not much is more effective.
I'm going with my safety glasses. I actually try and take care of mine so I can see out of them when I need them. The rig ones are always scratched to crap and useless. And I am totally the guy that brings em in on more calls than needed. You get one pair of eyes...