I have been using the Green Monster for over a month now. It works.
My stand by whistle is a Fox 40. Yes, judges bitch about them all the time. But, they work. And, I like the nifty color selection for the Fox 40 whistles (Red, White, Blue, Green, Yellow, Purple). My favorite lanyard - black cloth with gorgeous Navaho beadwork (worn only at the National) sports Blue and White Fox 40 whistles. My everyday competition lanyard - black leather - sports Red and Yellow Fox 40 whistles.
But, the Green Monster has about 50-75 yards on the Fox 40. When we are training and the dogs are getting to the end of a long blind, I typically switch to the Green Monster. It works, when the Fox 40 is dead.
I don't like the Green Monster otherwise. It is big and bulky. It clanks. I don't like the feel of it in my mouth ... it is too heavy. It works well when you push a lot of air through it. It doesn't work well with low volumes of air. I can't modulate its tone like I can with the Fox 40. But at the end of a long water blind in a heavy cross wind, I'll be switching to the Green Monster.