When I fish them, I simply dead drift them like a nymph.
I think they have limited utility, but are deadly in a narrow range of situations, including:
1) Newly stocked rainbows and brook trout. My wife and I stood in one place on the DH section of the North Mills in North Carolina and landed about 50 fish in under 2 hours on wine colored San Juans. And this was the second time she ever picked up a fly rod. Even though I was handing them out to guys that came along, there was a resentful mob forming by the time we started back to the car.
2) Wild and stocked rainbows in limestones or tailwaters with some tint or water color. I've done well a lot of places on San Juans with these conditions, including the Falling Spring, Green Spring (many years ago) and the Crooked River in the Oregon high desert.
3) Anywhere on a limestone stream with proximity to a hatchery. Especially for RT A wine or blood red San Juan was always the go-to fly for me in places like the section of Fishing Creek between the Humphrey Hole and the outflow at Tylersville, the section of Spring Creek below Benner Spring outflow and the smaller channel coming out of the hatchery at the Paradise, just about anywhere between the hatchery outflow and the high bridge below the fly shop on the Little Lehigh.
Basically anywhere a small egg fly would also work.
Other than that, they are excellent for bluegills in farm ponds. You have to have a pretty quick strike reflex though...