They are quite effective in some circumstances.
We used ours many times while cruising in the Sea of Cortez
where many of the anchorages are open roadsteads or otherwise exposed to a incoming swell. The diurnal wind
of onshore from noon to dark and offshore
from several hours after sunset to dawn meant that the boat
was frequently lying abeam of the prevailing swell.
The flopper stopper hung off the end of a 20' spinnaker
pole would reduce the roll by 50% or so. But, more importantly it would reduce the sharpness of the rolling. When the roll changed direction, as a swell would pass by, the boat
would gently change direction with the flopper stopper out but would change direction with a sharp jerk with no flopper stopper.
in La Cruz anchorage, Banderas Bay, near Puerto Vallarta
. We had been anchored there for several weeks and the boat had been moving in a circle around the anchor
on a daily basis. A big, 6' or more, swell built up from the WSW which was moving us toward shore. When I tried to haul in the 200' of anchor
chain in order to move to a more sheltered spot, I discovered the flopper stopper and the anchor float line were hopelessly tangled.
And we were slowly being pushed toward the shore break. Fortunately we had several experienced cruisers on board, for an afternoon Tequila Tasting Session, and we were able to sort out the problem and get the anchor up while Mirador got no closer than 200-yards from the big shore break.