I have the FA 28mm f2.8 soft lens the softness varies from f2.8 max to around f8 min, it performs as a normal auto lens from f8 to f22.
It is easy to use on modern digital cameras as the results can be instantly checked. The f2.8 setting is a little overdone for portraits for my taste but becomes highly controllable as the lens is stopped down and disappears at f8.
I am about to try it on micro 4/3 Panasonic G3 where it becomes effectively a 170mm, should be interesting.
The softness is more evident on highlights and works wonderfully on flowers in sunlight.
The results are superior to using filters or in post processing.