I find this tool very valuable as a dubbing rake, when I want dubbing from a hare's mask or other materials. When using it with a hare's mask, one obtains a nice blend of guard hairs and softer, differently colored underfur. It can be useful with synthetic materials, too, as another review mentioned.
However, I prefer to use a softer, smaller dubbing teaser on most (smaller) trout flies, such as a thin piece of Velcro (the rough part) mounted on a wooden coffee stirrer, reserving this tool for big stoneflies, other large dubbed nymphs and streamers, etc.