There are times they are useful but we don't use them much anymore. The general consensus is that the flappers work better than the spinning wing one. Opinions vary on this one, At least on this side of the pond.
Lots of post on this topic. Try search for Buster, Buster II, motion decoys, Flappers, Robo Crow.
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"If money can fix it, it ain't broke" The great theologian and my crow hunting partner AW.