I don't ever use a decoy when I'm the shooter, but almost always use them when I'm calling for someone else, especially kids. It helps to keep the attention away from the hunter. I prefer to keep the bird looking for me and not get hung up out of range because he already sees me and expects for the hen to come join him. All of my biggest birds have been when hunting without a decoy with the exception of my first merriams and this bird skirted the decoy, obviously nervous about it and made for a longer shot than usual.
Here in Idaho it's a bigger problem for me to get them out without being seen, not to mention having to carry one more things up and down these mountains. I had to pack that merriams back up 600 feet of elevation over a 2 mile hike, I'm just getting too old for that.
Decoys have their place, sometimes they help, sometimes they hurt. My dad won't hunt turkey without them and will put 3 or 4 of them out. When I used to hunt at the Kansas Governors one shot hunt, one of the guys in my group used 6 turkey decoys and had canada geese come into the green wheat field he was set up in.