Very popular, but don't seem to work for me. Diawl bach, PTN, Hares ear all fine, but crunchers - fish don't seem to care for them.
Floaters, intermediates, DI3 all had their go. For a "never off the cast" fly for some I must be using it at the wrong time, wrong way or just tied wrong (brownish hackle tail, two thirds pheasant/one third peacock, copper wire rib to help protect it from those fish

... and a greenwells hackle. I've seen Captains post on the original on another thread, maybe i need to go more claret on the peacock.
I'd put it down to a confidence thing but that can't be affecting just one fly on the cast as the Diawl Bach usually picks one or two up when their fished in a team.
How do you fish yours?