I've fished a bunch of evenings the last couple of weeks, and this fly has taken the majority of fish for me. Just about every evening on trout streams from the Northeast to Northern California, caddis hatch and trout feed on them. One of the primary caddis genera that hatches is the Hydropsyche, of which there are over a dozen species that hatch throughout the warmer months of the year. This pattern, designed by the folks at Blue Ribbon Flies in West Yellowstone, MT, imitates the emergers very well. So, tie some up, buy some, or have someone else tie some up for you, and fish them with confidence.
And sharpen your hooks.