It's hard to deny the effectiveness of decoys. However, the introduction of full-strut decoys has put a bad taste in a lot of hunters mouths. The modern decoys are much more realistic, but are generally more bulky and heavy. The main reason I don't use decoys all the time has more to do with the burden of carrying around the decoys all day rather than whether or not the decoys are actually effective.
Only a few times have I witnessed decoys actually spooking a turkey. The majority of these birds were spooked by a gobbler decoy, not a jake or hen decoy. My experience has been that they either come to the decoys or they completely ignore them.
Last season I was sneaking up on a couple of gobblers that were gobbling down in a hollar. After unsuccessfully calling to them from the ridge top, I decided to take out my jake decoy and try to set it up on the edge of the ridge where the gobblers could see him. I put the stake in the decoy and began crawling to the edge.
As I'm crawling to the edge, I look over and see two gobblers and a jake standing about 40 yards away. I'm literally on my hands and knees crawling in the wide-open woods with the decoy sticking straight up in front of me. Just when I think I'm busted, the gobblers start charging towards. At about 4 feet, one gobbler stopped and went into full strut. I'm still on my hands and knees looking down at the gobbler's spurs. My gun was laying on the ground and I knew if I moved it would be game over. (I never would have done this on public land)
With that said, I usually carry a single lightweight jake decoy in my vest at all times. If I'm hunting a big field, I'll throw out a few more decoys and maybe a full-strut jake decoy.