Have done very well fishing these little scud/euphasid patterns on occasion. Usually in Febuary and March. I only fish these when I find fish on a flat and they are spooky. These flies can account for a lot of 16-20 inch resident silvers if you can find them.
I had a couple days, about 3 years ago before I came down to CA for school, fishing the areaa south of the Agate pass bridge where we couldnt get close to these schools in a boat so I floated around in my tube and caught 20-30 fish in about 3 hours. Mix of both silvers and cutts.
They are very effective flies in the right situation but not a lot of fun to fish when you cant be sure where the fish are.