I have used them, I have called in a few foxes really close and a few coyotes but only within 200 yds. I don't think it was the calls fault, more the situation at the time. I have the Cass Creek Predator and Predator II, as well as the amplified speaker.
I am likely going to sell my 2 Cass Creek calls and my Johnny Stewart call this year, it might get me 1/5 of the way towards a FOXPRO FX3