Stopping down definitely makes it sharper, but I think it requires some experimentation with a subject to see if it works for you.
I shot a roll at f/8 in a park to get a feel for the out-of-focus rendering when it's stopped down beyond the obvious soft-focus range (f/3.5 - f/5.6):
https://www.flickr.com/photos/jeroenpulles/albums/72157670483141471
The 120mm focal length is just long enough to prevent perspective from distorting a portrait. It's closer to a normal/standard focal length than a "real" portrait lens. You could use a 1.4 converter if you prefer a longer focal length for portraits, though. Optimum length... I'd guess most people would prefer a longer lens for portraits. But then again, most people probably won't want to use a soft-focus lens either.
I have one very nice print of a picture of my father with this lens. This is not a lens for pixel-peeping, use it for print output, I think, from my limited experience with it.
kind regards,
Jeroen