If used properly, they are a huge asset. I have called in around 50 toms to decoys in the last 9 years. My son has been the lucky recipient for most of those. He is 14 and has shot 12 so far. Last year I called in 21 different toms on opening day for my son- he ended up with a triple. I prefer a single strutting tom with one hen. I use real turkey fans on my tom decoy and have him on a motion stake so he can move in the breeze. From 30+ yards, he looks like a real turkey. I set up the hen about 15 yards from the shooter and the tom at 18-20 yards. Most of the toms we have killed were squaring off to my tom decoy and many have attacked it.
As others have mentioned, safety has to be a huge consideration.
This is two real toms facing off my decoy.
My decoy took part of the spread...
Couple pics from years past. Without the decoys, I'm guessing his count would be considerably lower.
