I use one on my RZ in preference to the RZ 180 SF lens as I like the shorter focal length for portraiture and it's also nearly 200g lighter. It's not sharp until f8 because Mamiya has built in a degree of aberration to soften the image even without the diffusion discs.
However, I have to declare my preference for Mamiya's superb 6x4.5 soft focus lens (if only they made something similar for the 6x7 format) as this uses floating elements to create the varying softening effects - but always leaving the very centre of the image sharp. That solution is of course incredibly costly to manufacture and produces a very heavy lens. As for the 150 and 180 soft lenses for the Mamiya 67s, I'm never comfortable with having to unscrew the front elements to add the discs. It's not happened yet but I'm always nervous about foreign bodies getting into the lens space between the front and rear element housings as well as the risk of cross-threading in a moment of panic or loss of concentration.
One thing I can assure you - the RB67 150 SF lens can deliver excellent images - enjoy it.