I always thought that the step from a standard lens (50 mm or 55 mm) to the standard telephoto lens of 135 mm is quite a large gap. Therefore I prefer using a shorter telephoto lens around 100 mm, especially if I also have a 200 mm lens at hand.
This lens makes a very nice portrait lens, and it is much more compact compared to a 135 mm lens. You can also hand-hold it very well at 1/125 sec.
I was able to find this lens brand new still in the box, and it is an excellent performer. It also has hardly any distortion.
I never understood why Mamiya also offered a 105 mm SX lens since the difference in focal length is neglectable.