I've ever wanted to try a soft focus filter, of course for portraits to create a wonderful blurry touch. I bought an old one from Cenei and used it with my Minolta Dynax 8000i and the Sigma 3.5-4.5/28-80mm. The film was a very expired Kodak Gold 200.
The following pictures aren't good to show the effect of a soft focus filter, because the film was too expired. But the expired film and perhaps the filter created a cool vintage look.