I had one in my old freshwater tank. It was good at eliminating algae blooms.
So I believe that they can eliminate free-floating algae quite well.
Unfortunately, that cures a symptom, and essentially hides an underlying nutrient problem.
With my recent ich outbreak, I researched them a bit for disease control.
They appear quite useless against that, although might work against a few other pathogens.
Just not intense enough.
So I guess it depends on what your planned purpose is.