This Lens seems to be underestimated, maybe because the full aperture is mediocre. But at equal apertures, it is better than the excellent 105/2.5 AI in some respects (flatter field) and worse in others (bokeh). Optical formula is very similar to the 105/2.5 (Xenotar), which explains the similarity.
Then, there is the tolerance to highlights or backlight: striking, although I admit I never cared to make a direct comparison. And zero distortion, etc.
Of course it is a fat lens compared to the 2.5, but a compact one compared to all “modern” equivalents!
In any case, read Richard Haw’s review !