Used to HATE them...
We thought they were the dumbest things in the world. And the only experience we had with them was a young dog in for training that started retrieving decoys we guess because she had grown up with dokkens. I think that's the key point.
HOWEVER... We got some when the ESPN thing started a couple of years ago, just to practice with and make sure the dog was used to them.
In the meantime, here are things we've used them for (although we don't use them very often):
1) Dogs that are sloppy on birds, not yet through FF, but you want to give them a bird-like feel to what they're retrieving
1b) Right after FF to help the dog transition more easily to bird if you think they'll have problems.
2) Cheating singles, where we'd normally use bumpers (because birds would get nasty with repeated throws, esp in summer). I must say that they really do make a difference to the dogs. They think they're a bird at least until they get to them!
3) As a transition to birds for dogs who aren't very birdy or just weren't introduced to birds early enough in their life.
We don't use them often. There is NO substitute for the real thing, which is what we use every day in training.
We were at an outdoor show and I went over to a retriever supply booth and took one back and showed it to one of the dogs we had there. As soon as she even SAW it, she went nuts (it was just a little dokken dove). She loved it. I bought it just for fun.
I wouldn't invest money in more than a few of them to have around for the above reasons, though. I wouldn't raise a dog on them. And I wouldn't use them every day in training. But for the things I use them for, I think they're fantastic.
-Kristie