How well it does depends a lot on where you put the gun. I've heard from lots of people, both people I know and on here, that they installed it, it performed poorly, and they removed it. I've installed probably 25, and I always mount the gun on the wall directly across from the meter box and hang the foam brush from the center of that wall, so customers have to walk past the foam gun to get to the brush. They have no trouble finding and using it. About a third of my customers use it. $200 in chems last six months. I'd have to roughly guesstimate that it pays for itself over again every year.
I watched at one guy's wash one day where he mounted the foam gun holder catacorner from the meter box. His customer selected it, looked around for it for half a minute, then finished without ever using it. He's one that ended up removing it for lack of use.
Going by what other forum members have said, I think I'd do in-bay dryers before trifoam.